Rex Tillerson, the U.S. Secretary of State, arrived in Beijing on Saturday for a face-to-face meeting with a China official who last week likened the U.S., South Korea and North Korea to speeding trains ready to hit each other.

Tillerson’s visit followed his remarks in South Korea on Friday in which he warned that pre-emptive military action against North Korea might be necessary if the threat from their weapons program reaches a level “that we believe requires action.”

China, the North’s biggest source of diplomatic support and economic assistance, has yet to respond to his remarks, although Beijing has called repeatedly for steps to reduce tensions.

Chinese Foreign Minister Wang Yi, with whom Tillerson was due to meet on Saturday afternoon, spoke about the tension between the countries. He said, “The question is: Are the two sides really ready for a head-on collision?” Wang told reporters. “Our priority now is to flash the red light and apply the brakes on both trains.”

Wang said North Korea could suspend its nuclear and missile activities in exchange for a halt in joint U.S.-South Korea military drills, a proposal swiftly shot down by U.S. Ambassador to the United Nations, Nikki Haley, who said Washington has to see “some sort of positive action” from North Korea before it can take leader Kim Jong Un seriously.

Tillerson: Nothing is off the table in dealing with N. Korea

Tillerson’s comments in Seoul that “all of the options are on the table,” including possible military action, are likely to be deeply disconcerting to Beijing, which fears that a collapse of Kim’s regime would send waves of refugees into northeastern China and land South Korean and American forces on its border.

China has agreed reluctantly to U.N. Security Council resolutions sanctioning North Korea, while calling for renewed dialogue under the Beijing-sponsored six-nation format that broke down in 2009.

In a further sign of its frustration with Pyongyang, China last month banned imports of North Korean coal for the rest of the year, potentially depriving Kim’s regime of a key source of foreign currency.

Past U.S. administrations have considered military force because of North Korea’s development of nuclear weapons and ballistic missile to deliver them, but rarely has that option been expressed so explicitly as by Tillerson.

North Korea has accelerated its weapons development, violating multiple Security Council resolutions without being deterred by sanctions. The North conducted two nuclear test explosions and 24 ballistic missile tests last year. Experts say it could have a nuclear-tipped missile capable of reaching the U.S. within a few years.

China has stridently opposed the deployment of a U.S. missile defense system to South Korea, saying its X-band radar can peer deep into China to monitor flights and rocket launches. The U.S. says it’s a system focused on North Korea. China sees it as a threat to its own security.

Tillerson’s visit to Beijing is the final stop on his three-nation swing through Northeast Asia, which began in Japan. State Department officials have described it as a “listening tour” as the administration seeks a coherent North Korea policy, well-coordinated with its Asian partners.

In Beijing, he is also expected to discuss China’s claim to virtually the entire South China Sea, including its building of islands atop coral reefs, complete with airstrips and military installations.

During his confirmation hearings in January, Tillerson compared China’s island-building and deployment of military assets to Russia’s 2014 annexation of Crimea, and suggesting China’s access to the island should not be allowed.

While President Donald Trump during his campaign pledged to slap 45 percent tariffs on imports from China and label the country a currency manipulator, there has been little sign of his doing either. His pick for U.S. trade representative, Robert Lighthizer, has said he would use a “multi-faceted approach” to cracking down on Chinese trade abuses.

Tillerson’s trip is also expected to highlight the Trump administration’s lack of concern with human rights abroad, formerly a key element of U.S. policy toward China and a major irritant for Beijing.

In a departure from past practice, Tillerson skipped the launch of an annual report on human rights last week that cited numerous abuses by China. He has also said the U.S. would not continue participating in the U.N. Human Rights Council unless it undergoes “considerable reform.”

The Associated Press contributed to this report