“One of the important reasons behind the improvement of the relationship between China and South Korea is that the two countries have reached a consensus that no war should be allowed to occur on the Korean Peninsula,” said Feng Zhang, a fellow in international relations at Australian National University. “Here, China and South Korea have a perfect common interest, even though China was not directly involved in the inter-Korean talks.”

One of China’s senior experts on North Korea said it was not clear whether Mr. Kim was really offering anything new, and that it could turn out that the North was taking advantage of Mr. Moon’s eagerness for dialogue.

“North Korea believes South Korea is the weakest link in the U.S. alliance,” said the expert, Zhang Liangui, a professor of international studies at the Central Party School of the Communist Party in Beijing. “The United States and the outside world will continue to doubt the true purpose of North Korea, because they do not know whether this is just a diplomatic strategy or a fundamental policy shift.”

China has supported United Nations sanctions against the North over its nuclear program — like sending its workers home, curbing shipments of oil and cutting off coal exports — though it has stopped short of imposing unilateral financial sanctions, as proposed by Washington. Mr. Trump said on Tuesday that China had been “very, very strong and very biting” in its support for sanctions.

Beijing’s cooperation with Mr. Trump on the issue has made relations between North Korea and China so poisonous that the North has refused to allow a visit by Kong Xuanyou, a senior diplomat appointed by Beijing to deal with Pyongyang.

A close look at the language used on Tuesday by Chung Eui-yong, one of the top South Korean envoys who met with Mr. Kim, indicates that the North Koreans are offering a formula similar to what has been rejected in the past, said Evans J. R. Revere, a former State Department official who participated in past talks on the North.