IS held the ruins and the nearby city, known locally as Tadmur, for 10 months after seizing it for the first time in May 2015. It blew up temples, burial towers and the Arch of Triumph, believing shrines and statues to be idolatrous.
They also destroyed the Temple of Bel – the great sanctuary of the Palmyrene gods – which had been one of the most important religious buildings of the 1st Century AD in the East.
Site contains monumental ruins of great city, once one of the most important cultural centres of the ancient world
Art and architecture, from the 1st and 2nd centuries, combine Greco-Roman techniques with local traditions and Persian influences
More than 1,000 columns, a Roman aqueduct and a formidable necropolis of more than 500 tombs made up the archaeological site
More than 150,000 tourists visited Palmyra every year before the Syrian conflict
The militants were then forced out by a Russian-backed government offensive in March 2016, but regained control while pro-government forces where focused on battling for the city of Aleppo late last year.
In January, satellite images revealed that the group destroyed the tetrapylon – a group of four pillared structures which were mainly modern replicas – and part of the Roman Theatre.
Meanwhile, the US said positions held by the their allies of the Syrian Arab Coalition were bombed by Russian and Syrian aircraft near the IS-held town of al-Bab, in northern Syria.
“I believe they thought were held by ISIS, yet they were actually – on the ground – were some of our Syrian Arab coalition forces,” Lt Gen Stephen Townsend told reporters, using an acronym for IS.
The US ambassador to the UN, Nikki Haley, said: “It is a sad day on the Security Council when members start making excuses for other member states killing their own people.
“They put their friends in the Assad regime ahead of our global security… the world is definitely a more dangerous place.”
UK Ambassador Matthew Rycroft said: “Not taking action against chemical weapons’ use undermines confidence in the international community’s ability to tackle flagrant violations of international law – and undermines the trust of Syrians affected by these horrific attacks.”
French Foreign Minister Jean-Marc Ayrault said Russia bore a “heavy responsibility toward the Syrian people and humanity as a whole”.
The Syrian Observatory said it could not determine if the strikes, in the countryside of Idlib province, were carried out by the US-led coalition or Russia.
Among the dead were leading members of Jabhat Fateh al-Sham, which was holding a meeting there, the Observatory added, without identifying them. Many others were wounded, it said.
Abu Anas al-Shami, a Jabhat Fateh al-Sham spokesman, said in a statement the attacks were carried out by the US-led coalition.
“The headquarters targeted by the international coalition a short time ago are a main headquarters for that area and contains a number branch offices, leading to the killing of the brothers,” he said in a statement quoted by Reuters news agency.
Members of the group are currently operating as part of a rebel alliance that controls Idlib province, the main rebel stronghold after eastern Aleppo was retaken by Syrian government forces last month.
As well as the Jabhat Fateh al-Sham, the ceasefire deal excludes so-called Islamic State (IS) and the Syrian Kurdish Popular Protection Units (YPG) militia, according to the Syrian army, but the exclusion is disputed by the rebels.
Fighting between government and rebel forces has been reported in parts of Syria despite a nationwide truce coming into force early on Friday.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said there had been fierce clashes and air strikes in northern Hama province.
It added that rebel-held Wadi Barada near Damascus was also bombarded. But the military denied doing so.
There has been no comment from Turkey and Russia, which brokered the truce.
The rival jihadist groups, Islamic State (IS) and Jabhat Fateh al-Sham, and the Kurdish YPG militia are excluded from the initiative, which is aimed at restarting peace talks in Kazakhstan’s capital Astana next month.
Reports of clashes emerged within hours of the truce starting at midnight local time on Friday (22:00 GMT on Thursday).
The Syrian Observatory said government warplanes had carried out 16 air strikes on rebel-held areas in the northern countryside of Hama province during the day on Friday.
The Local Co-ordination Committees (LCC), an opposition activist network, said the town of Halfaya had been targeted.
The Syrian Observatory and LCC also reported fighting in Wadi Barada, a valley in the mountains north-west of Damascus.
They said helicopters had attacked the village of Basima and positions held by rebels and allied jihadists from Jabhat Fateh al-Sham, which was known as al-Nusra Front until it formally broke ties with al-Qaeda in July.
But a Syrian military media unit denied that troops had shelled Wadi Barada and accused the opposition of aiming to show it was not abiding by the truce.
The new kingmaker – Lyse Doucet, BBC chief international correspondent
This deal was declared before it was done and dusted. Seven groups said to have signed up include Ahrar al-Sham, which Moscow and Damascus have always described as terrorists. Ahrar al-Sham says it has “reservations”. Do they have anything to do with backers like Saudi Arabia and Qatar?
But a new top table has been forged before a new US president enters the scene. Russia is confirmed as the foreign force which matters. Turkey displaced the US as kingmaker on the other side. It has bargaining chips and, most of all, wants to stop the sway of Syrian Kurdish forces, who are US allies.
Many opposition fighters will welcome a pause after their stinging defeat in Aleppo. But they and Turkey still want President Assad to step down. That conflicts with Iran, the other key player, as well as Mr Assad’s own circles. But that’s for the next round in this new great game which could be talks in Astana, in Russia’s orbit.
The UN expressed concern about the fighting in Wadi Barada on Thursday, saying combatants were deliberately targeting and damaging springs used to supply some four million people in the Damascus area with drinking water.
The LCC also reported on Friday that government shellfire had caused casualties in rebel-held Douma, in the Eastern Ghouta region outside Damascus.
Abdulkafi Alhamdo, a teacher who was living in a rebel-held enclave in the northern city of Aleppo before being evacuated as part of a deal negotiated by Turkey and Russia earlier this month, said he was not optimistic.
“I can sleep a bit better and not wake up during the night in fear. But we have also experienced many ceasefires in the past and they don’t last,” he told the BBC.
“I believe [government forces] use that time to prepare their troops, and fix their planes and then they just target us again.”
Meanwhile, Turkish military officials said Russian aircraft had carried out three air strikes against IS militants around the northern town of al-Bab.
The strikes appeared to be the first Russian support for a Turkish-backed rebel offensive aimed at recapturing the last IS stronghold in Aleppo province.
Russia has carried out an air campaign against President Bashar al-Assad’s opponents since September 2015, while Turkey supports the rebellion.
Who is included by the truce agreement?
On the one side, Syrian government forces, allied militias and the Russian military.
On the other, a loose alliance of moderate rebel factions that operate under the banner of the Free Syrian Army (FSA), plus several other groups.
The Russian defence ministry named seven “moderate opposition formations” included in the truce as Faylaq al-Sham, Ahrar al-Sham, Jaysh al-Islam, Thuwwar Ahl al-Sham, Jaysh al-Mujahidin, Jaysh Idlib and Jabhah al-Shamiya.
Ahrar al-Sham, which said it had “reservations” about the deal, and Jaysh al-Islam are Islamist groups that Russia has previously described as terrorist organisations.
Who is not included?
The jihadist groups Islamic State and Jabhat Fateh al-Sham, “and the groups affiliated to them”, are not part of the agreement, according to the Syrian army.
Jabhat Fateh al-Sham said on Friday it would continue to fight Bashar al-Assad, with a spokesman for the group saying the political solution under the truce would “reproduce the criminal regime”.
Members of the group are currently operating as part of a rebel alliance that controls Idlib province.
The FSA also said the deal did not include the Kurdish Popular Protection Units (YPG). The militia, which has captured large swathes of north-eastern Syria from IS with US support, is designated a terrorist organisation by Turkey.
Where does it cover and what are the terms?
It is nominally nationwide, although that really only covers the areas where the sides who have signed up have a presence – western Syria.
Swathes of central and eastern Syria are under IS or Kurdish control.
Under the terms of the deal, talks on a political solution to end the civil war should begin within a month of the start of the truce and would be held in Kazakhstan.
On the one side, Syrian government forces, their factional allies and the Russian military.
On the other, the Free Syrian Army (FSA), a loose alliance of several moderate rebel factions, plus other groups under the High Negotiations Committee (HNC), the umbrella group representing Syria’s political and armed opposition factions.
FSA spokesman Osama Abu Zaid said there were 13 armed opposition factions in all who had signed up.
The Russian defence ministry listed seven of the main rebel groups included in the truce as Faylaq al-Sham; Ahrar al-Sham; Jaysh al-Islam; Thuwwar Ahl al-Sham; Jaysh al-Mujahidin; Jaysh Idlib and al-Jabhah al-Shamiyah.
Ahrar al-Sham and Jaysh al-Islam (Army of Islam) are the key names, neither of them part of the FSA.
Who is not included?
Jihadists. So-called Islamic State (IS) “and the groups affiliated to them” are not part of the agreement, Syria’s army confirmed.
It also said Jabhat Fateh al-Sham (formerly the Nusra Front) was excluded. However, some rebel officials told Reuters it was part of the deal, giving a hint of the complications that lie ahead.
This is because JFS is intrinsically linked in Idlib province to groups that have signed up to the truce.
The FSA also said that the deal did not include the Kurdish Popular Protection Units (YPG).
The YPG, along with other Kurdish militias, controls a large area of northern Syria up the Turkish border. It is regarded by Turkey as a terrorist organisation and an extension of the banned Kurdistan Workers’ Party (PKK).
What are the terms and where does it cover?
It is nominally nationwide, although that really only covers the areas where the sides who have signed up to the truce have a presence.
Looking at the map, there are large swathes under both jihadist and Kurdish control.
One area that is included is the rebel-held area of Ghouta in eastern Damascus, where government forces have been advancing in recent months.
Announcing the deal in Moscow, Mr Putin said there were three key points:
Ceasefire between the two sides
Measures for overseeing the truce
An agreement to start peace talks
Under the terms of the deal, the peace talks would begin within a month of the ceasefire taking effect – and holding – and would be held in the capital of Kazakhstan, Astana.
The Kurdish YPG would not take part in the peace talks, the FSA said.
How did the ceasefire come about and can it hold?
Previous ceasefire initiatives this year brokered by the UN, or the US acting with Russia, quickly collapsed.
But earlier this month, Russia and Turkey negotiated a ceasefire in Syria’s second city, Aleppo, that led to tens of thousands of rebel fighters and civilians being evacuated from an enclave besieged by government forces.
The fact that the rebels have been losing ground may help this truce.
The HNC admitted on Thursday that, because of the rebels’ limited resources against government forces and their allies, it was “not possible to continue” the fight.
What has the reaction to the deal been?
Mr Putin himself described the deal as “fragile”.
He said Russia would cut its military presence in Syria but made it clear Moscow would “continue fighting international terrorism and supporting the Syrian government”.
Syrian Foreign Minister Walid Muallem said there was now a “real chance of finding a political solution” but that the rebels taking part must distance themselves from IS and former al-Qaeda fighters.
Turkish President Recep Tayyip Erdogan said a “window of opportunity” had been created that “should not be wasted”. He vowed to continue fighting IS and “terrorist groups”.
Difficulties lie ahead in peace talks. Turkey has already called for the Lebanese Hezbollah movement – linked to Syria’s ally Iran – to leave Syrian soil, which is sure to cause problems.
UN envoy for Syria, Staffan de Mistura, welcomed the new deal, saying he hoped it would save lives and improve aid delivery.
“These developments should contribute to inclusive and productive intra-Syrian negotiations to be convened under UN auspices on 8 February,” he said.
The US state department said the deal was a “positive development” which it hoped would be “implemented and fully respected by all parties”.
One rebel commander told Reuters: “This time I have confidence in its seriousness. There is new international input.”
The FSA said it would abide by the truce but would retaliate against violations by government forces and their allies.
The FSA’s Osama Abu Zaid said it had so far had no direct talks with the Syrian government and still insisted that President Assad would have no place in the future of Syria.
The evacuation of the opposition-held part of eastern Aleppo was part of a deal brokered by Russia and Turkey.
In return, residents of the government-controlled towns of Foah and Kefraya in Idlib province, besieged by rebels, would also be removed.
Heavy snow, strong winds and the poor state of vehicles had slowed the evacuation, Red Cross officials said. Thousands of people were forced to wait for hours in sub-zero temperatures with little food or drink.
In a statement on Syrian TV, the army announced the “return of security to Aleppo”.
“This victory represents a strategic change and a turning point in the war against terrorism on the one hand and a crushing blow to the terrorists’ project and their supporters on the other hand,” it said. The government usually refers to the rebels as “terrorists”.
The statement said the victory was a further incentive for the army to carry on fighting to “eradicate terrorism and restore security and stability to every span on the homeland”.
Ahmed Qorra Ali, part of the rebel group Ahrar al-Sham, told AFP that “the last convoy has left the rebel-controlled area”.
Aleppo was once Syria’s largest city and its commercial and industrial hub before the uprising against Mr Assad began in 2011.
For much of the past four years it was divided roughly in two, with the government controlling the western half and rebels the east.
Troops finally broke the deadlock this year with the help of Iranian-backed militias and Russian air strikes, reinstating a siege on the east in early September.
After breaking through the rebels’ defensive lines in mid-November, they quickly advanced and had seized all but 2.6 sq km (1 sq mile) by the time a ceasefire was brokered.
Earlier, Russia’s Defence Minister Sergei Shoigu announced that its warplanes had carried out 18,800 sorties since launching an air campaign against Mr Assad’s opponents in September 2015.
In total, they had “liquidated 725 training camps, 405 weapon factories and workshops, 1,500 pieces of terrorist equipment, and 35,000 fighters”, he said.
Human rights groups have accused the Russian and Syrian air forces of committing war crimes in Aleppo, alleging that they killed hundreds of civilians this September and October alone, deliberately targeted medical facilities, and used indiscriminate weapons such as cluster and incendiary munitions.
Evacuations have resumed from east Aleppo, with buses and ambulances leaving rebel areas of the Syrian city, a UN official says.
At least 350 people reportedly left rebel enclaves in convoys late on Sunday, towards government territory.
Earlier, buses sent to take people out of government-controlled areas, besieged by rebels, were set alight, halting the latest evacuation deal.
Thousands are waiting to leave east Aleppo in desperate conditions.
The UN Security Council is said to have agreed a compromise to allow UN monitoring of the operation. Russia earlier rejected a French-drafted plan to send UN officials to east Aleppo as “a disaster”.
“We expect to vote unanimously for this text,” said US Ambassador to the UN Samantha Power.
The Security Council meeting is due to start at 09:00 (14:00 GMT) in New York.
Initial efforts to evacuate the last rebel-held enclaves in the city collapsed on Friday, leaving civilians stranded at various points along the route out without access to food or shelter. Bombardment of east Aleppo has left it virtually without medical facilities.
Despite further setbacks on Sunday, buses and ambulances began moving out of the area after nightfall.
“Evacuations are on,” the UN official said in an email message to Reuters news agency, adding that the first people left east Aleppo at around 23:00 local time (21:00 GMT).
Five buses carrying evacuees arrived in rebel-held Khan al-Assal, AFP news agency quoted Ahmad al-Dbis, who heads a team of doctors coordinating evacuations to the town, as saying.
From Khan al-Assal, the evacuees are expected to travel to government held parts of Aleppo and Idlib provinces.
Pro-government forces previously demanded that people must be allowed to leave the mainly Shia villages of Foah and Kefraya in Idlib province, besieged by rebels, in order for the evacuation of east Aleppo to restart.
Some 1,200 people were due to be taken out of the former rebel enclave in return for a similar number moved out of the two government-held villages, Foah and Kefraya.
However, it is not thought any of the some 2,000 people believed to live in the two villages left on Sunday.
Earlier on Sunday, armed men set fire to at least five buses that were about to transport the sick and injured from the villages.
Several reports said the Jabhat Fatah al-Sham rebel group, formerly al-Qaeda-linked Nusra Front, was responsible. But Hezbollah, the Lebanese militant group fighting alongside Syria’s government, said the blaze started during fighting between the jihadist Fatah al-Sham and another Islamist rebel group that supported the evacuations.
The jihadist groups have not commented on the attack.
However, the Free Syrian Army, a more moderate rebel faction, condemned it as a “reckless act endangering the lives of nearly 50,000 people” in east Aleppo.
Among the people waiting to leave eastern Aleppo are sick and wounded children, said the children’s charity Unicef.
Some young children have been forced to leave without their parents, the charity said, and hundreds of vulnerable children remain trapped.