BEIJING -- North and South Korea have scheduled a direct meeting between leaders Kim Jong Un and President Moon Jae-in for late April, but as CBS News correspondent Ben Tracy reports, the main event for Kim is still a potential summit with President Trump. The two men are expected to meet by the end of May.

Tracy asked U.S. Ambassador to China Terry Branstad on Thursday about the surging efforts to convince Kim to give up his nuclear weapons. The U.S. envoy told CBS News that while goal may be clear, the end result cannot be taken for granted.

"I think we have to be very careful in dealing with him," Branstad told CBS News, noting that Kim may be, "feeling the heat of the U.N. sanctions and the pressure that is being placed on him, and maybe it means he's changing his attitude a little bit."

Tracy notes that it has been four months since Kim tested a missile. Instead, he's launched a diplomatic offensive, successfully getting the presidents of both South Korea and the United States to agree to meet with him.

Branstad told CBS News that the goal of President Trump's expected meeting -- which has not yet been publicly confirmed, or even mentioned by Kim's regime -- "is indeed to get a verifiable denuclearization program."

"This is something that has to be real and verifiable," he cautioned, "and something they won't renege on as they did in the past."

Kim left North Korea for the first time as its leader earlier this week for an unannounced visit to Beijing, where he received a very warm welcome from Chinese President Xi Jinping.

Long-time allies China and North Korea had been at odds in over the past year over China's support for tough U.N. sanctions against the Kim regime. The visit was seen by many as a very public display that the two countries are still close ahead of Kim's upcoming spring summits.

As the U.S. keeps up pressure to isolate North Korea, Tracy asked Branstad whether the Chinese welcoming Kim so cordially might have helped legitimize him ahead of the crucial meetings with Moon and Mr. Trump.

"Remember the Chinese are very good hosts," the ambassador said. "So I wouldn't read too much into that."