In a phone call with U.S. President Donald Trump last month, Chinese President Xi Jinping called for setting up a new four-way security framework involving China, North Korea, South Korea and the United States to ensure peace on the Korean Peninsula, diplomatic sources close to relations between Washington and Beijing said.

The proposal on March 9 included the idea of signing a peace treaty among the four countries, apparently to replace the armistice that halted, but technically never ended, the 1950-1953 Korean War, said the sources, who spoke late last month on condition of anonymity. The four nations were the main parties in the Korean War.

It is unclear whether the four-way framework is being viewed by China as an alternative to the long-stalled six-party denuclearization talks, which also involve Japan and Russia.

During the talks with Trump, Xi made no reference to Japan, the sources said.

His remarks could signal China’s willingness to lead negotiations on the nuclear issue after North Korean leader Kim Jong Un’s meetings with Moon and Trump.

While welcoming Trump’s plan to meet with Kim by the end of May, Xi said Beijing’s involvement in the denuclearization talks with Pyongyang is vital, the sources said.

Trump did not give a clear answer to Xi’s proposal, the sources said.

Xi’s remarks suggesting China’s desire to maintain a pivotal role in dealing with the nuclear issue came amid North Korea’s dramatic easing of tensions with the U.S.

Xi said he regards the Trump-Kim meeting as a turning point in reining in the North’s nuclear program, but stressed that long-term efforts with China’s engagement are needed to achieve that goal, the sources said.

Xi also said China is committed to supporting a process to verify that North Korea has actually disarmed, the sources said.

Kim and Xi held surprise talks on March 26, during which, according to Chinese official media, he said Pyongyang is committed to denuclearizing the peninsula.