A surprise meeting between President Moon Jae-in of South Korea, right, and Kim Jong-un, the North Korean leader, on Saturday appeared to produce some progress toward a summit meeting between President Trump and Mr. Kim.CreditPresidential Blue House, via Getty Images

WASHINGTON — Technical and diplomatic experts from the United States have crossed into North Korea to meet with their counterparts, American officials said on Sunday, as part of an intense behind-the-scenes effort to resurrect the possibility of a summit meeting between the two countries' leaders.

Led by Sung Kim, a veteran diplomat with years of experience negotiating with North Korea, the team is seeking detailed commitments from Kim Jong-un, the North Korean leader, about his regime's willingness to abandon its nuclear weapons program. News of the delegation was first reported by The Washington Post.

In Singapore, White House officials said, Joe Hagin, a deputy White House chief of staff, is leading a separate delegation to work out the logistics of a meeting between President Trump and Mr. Kim: when the various meetings would take place, how much would be open to the press, which officials would be in the negotiating rooms, how to handle security concerns.

Such issues would typically be handled by a well-established diplomatic process of lower-level negotiations that usually takes months, if not years.

But Mr. Trump short-circuited that process when he abruptly accepted in March an invitation to meet with Mr. Kim.

Now, after just as abruptly canceling the summit meeting scheduled for June 12 in Singapore, Mr. Trump has — wittingly or not — set in motion a more normal set of discussions to lay the groundwork for an agreement about North Korea's nuclear weapons program ahead of a decision on whether to hold a meeting between the two leaders after all.

The timeline is still extraordinarily condensed. Mr. Trump's repeatedly stated desire to keep June 12 as a possible date for a summit meeting means that officials on both sides are racing to see if work can be completed in a matter of days. Veteran negotiators said it remained unclear whether the two sides could complete enough work to make a meeting possible.

"The president says he's not going to go until there is substantial agreement. The question is, Is there time to reach that kind of agreement?" said Joseph Y. Yun, a former chief North Korea negotiator at the State Department, who retired in part because of his frustration with his agency's diminished role. "Right now, the summit is kind of teetering on whether we make progress on those things."

American officials have said the discussions are moving along well, expressing optimism about the possibility of a summit meeting that is clearly intended for an audience of Mr. Trump.

In brief remarks to reporters on Saturday night, the president said the lower-level negotiations are "going along very well," though he added his usual caveat: "We'll see what happens." On the Korean Peninsula, a surprise meeting between Mr. Kim and President Moon Jae-in of South Korea also produced some progress toward a meeting.

Mr. Moon said Mr. Kim wanted to discuss "complete denuclearization of the Korean Peninsula" with Mr. Trump.

"What is not so clear to him is how firmly he can trust the United States' commitment to ending hostile relations and providing security guarantees for his government, should it denuclearize," Mr. Moon said after the meeting on the North Korean side of Panmunjom, a "truce village" inside the Demilitarized Zone that separates the two Koreas.

The answer to that question may hinge on the lower-level discussions going on between representatives of the two countries, a fraught process that can sometimes dissolve into disagreement and at other times produce halting progress toward a presidential meeting.

Administration officials say they are under no illusions that the team now in North Korea can negotiate the details to begin to dismantle the sprawling nuclear, missile and biological weapons programs in North Korea — all of which were part of the objective Secretary of State Mike Pompeo laid out recently for the talks. In the case of the Iran deal, a detailed plan struck in 2015 that Mr. Trump abandoned this month as insufficient, the negotiations took more than two years.

But they can negotiate language and a timetable that Mr. Kim and Mr. Trump can agree on, a framework for further negotiations. That alone would be a major accomplishment, as North Korea has rejected the idea of rapid denuclearization, and wants a step-by-step approach.

The United States would also have to agree to changes it would make, perhaps opening talks on a peace treaty that would formally end the Korean War. Another possible concession would be to provide security guarantees to the North that go beyond previous presidents' pledges that the United States would not seek to overthrow the current government.

But for now, veteran diplomats said the negotiating teams from the United States have the kind of experience needed to at least work out the details needed for a summit meeting.

Mr. Kim, the former United States ambassador to South Korea, is a career diplomat who participated in the six-country negotiations with North Korea when George W. Bush was president. News of any progress his team is making will likely be sporadic, given the lack of secure communications from inside North Korea, officials said.

Mr. Yun said the goal for Mr. Kim was likely to be developing a set of documents, agreed on by both sides, that detail the three steps that North Korea is willing to consider taking toward elimination of its nuclear weapons program.

The first step, he said, is a declaration of how far the North Koreans are willing to go in unwinding their weapons program. The second is deciding how and when the North Koreans would provide an accounting of that process to the United States. And the third step is determining how the United States would verify those claims.

"It's a good group," Mr. Yun said. "It's a technical group. It's an expert group, and they know the issues. They know what needs to be done."