SEOUL, South Korea — The United States and North Korea have been negotiating with “will and sincerity” over the details of the planned talks between President Trump and the North’s leader, Kim Jong-un, President Moon Jae-in of South Korea said on Wednesday.

Mr. Trump said on Monday that American officials had been talking directly with the North Koreans to prepare for his meeting with Mr. Kim, which he said would probably take place in May or early June. Mr. Moon said the two sides were discussing where to hold the meeting, among other issues.

”I hear that the United States and North Korea are preparing for the summit with both resolve and sincerity, holding detailed negotiations over the time, venue and agenda,” Mr. Moon’s office quoted him as saying on Wednesday, during a meeting with officials preparing for his own talks with Mr. Kim on April 27.

“I am expecting the North Korea-United States summit to produce significant steps toward denuclearizing the Korean Peninsula and establishing permanent peace here,” Mr. Moon said.

Both Mr. Moon and Mr. Trump hope to persuade Mr. Kim to dismantle Pyongyang’s nuclear weapons program, which made dramatic advances last year even as the American and North Korean leaders exchanged belligerent threats. Mr. Kim abruptly changed tack in January with diplomatic overtures, first to South Korea and later to Mr. Trump, who stunned the world by abruptly agreeing to talk directly to Mr. Kim.

South Korean and American officials have said that the North expressed willingness to discuss denuclearizing. But it is unclear what Mr. Kim would want in return and whether Washington would meet his demands.

No American president has ever met with a North Korean leader. The two countries do not have diplomatic relations, which has led to complicated logistical issues around the summit meeting — starting with where it will be held.

It is unclear whether Mr. Kim would be daring enough to visit Washington, which North Korea has for decades accused of plotting to invade it. And Mr. Kim’s private jets are said to lack the fuel capacity for a nonstop flight to Washington from Pyongyang.

The idea of a summit meeting in Pyongyang makes some American officials cringe, envisioning how North Korean propagandists would depict it as Mr. Trump paying homage to Mr. Kim. Some officials have also expressed reluctance to hold the talks in South Korea or at Panmunjom, the “truce village” on the inter-Korean border where Mr. Kim and Mr. Moon will hold their talks; the American officials said they would prefer a neutral site that did not highlight the South’s role in facilitating the talks.

News reports in South Korea and the United States have mentioned Geneva and the Mongolian capital, Ulan Bator, as possible venues.

Mr. Moon has tried to be a mediator between Mr. Kim and Mr. Trump. His envoys met with the North Korean leader in Pyongyang in March, and went to Washington soon afterward to tell Mr. Trump that Mr. Kim wanted to talk.

According to South Korean officials and analysts, Mr. Moon hopes for a “comprehensive deal” in which Mr. Kim would commit to dismantling his nuclear arsenal and Mr. Trump would reciprocate by offering security guarantees for the North, including normalized ties with Washington.

But implementing such a deal would probably be extremely complicated and protracted. Past talks between North Korea and the United States have had similar aims, but they all collapsed in the early stages, over disputes about freezing the North’s nuclear activities and how such a freeze would be verified.

North Korea, as it has in the past, insists on a “phased” and “synchronized” implementation of any denuclearization deal. According to former South Korean officials who have dealt with the North, it fears that any deal it signs with Washington could come to an end after a change in American administrations. So rather than surrender its nuclear facilities up front, it wants incremental steps, matched with corresponding incentives from the United States.

For their part, American officials have said that North Korea has never been sincere in dealing with them, using negotiations to buy time while persisting in clandestine nuclear weapons development. Some hard-liners in Washington, like Mr. Trump’s new national security adviser, John R. Bolton, have demanded a quick dismantling of the North’s nuclear weapons program, suspecting that the North only wants to ease the tough international sanctions against it in exchange for a temporary, and deceptive, freezing of its nuclear program.

South Korea hopes that North Korea and the United States will agree on a road map toward denuclearization and quickly implement key steps before Mr. Trump’s term ends in January 2021, according to scholars advising Mr. Moon’s government. They said that a key challenge for Mr. Moon would be to persuade Mr. Kim and Mr. Trump to exchange key trust-building steps soon after they meet, like granting inspectors unfettered access to the North’s nuclear facilities and setting up liaison offices in each other’s capitals.

”We will have to make preparations for the South-North Korean summit meeting to serve as a guide for the North Korea-United States summit,” Mr. Moon said on Wednesday.

Mr. Kim met with President Xi Jinping of China in Beijing last month, in a bid to mend the North’s strained ties with its traditional communist ally before his meetings with the South Korean and American leaders.

In a sign of warming relations between Pyongyang and Beijing, a senior Chinese diplomat, Song Tao, will bring an art troupe to North Korea this week, North Korean and Chinese state media reported on Wednesday. The Chinese delegation will arrive in North Korea on Friday to attend an international arts festival, the report said.