SEOUL — 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 Wednesday.

Trump said Monday that US officials had been talking directly with the North Koreans to prepare for his meeting with Kim, which he said would probably take place in May or early June. 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 will and sincerity, holding detailed negotiations over the time, venue and agenda,’’ Moon’s office quoted him as saying Wednesday, during a meeting with officials preparing for his own talks with 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,’’ Moon said.

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

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

No sitting US 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 Kim would be daring enough to visit Washington, which North Korea has for decades accused of plotting to invade it. And 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 US officials cringe, envisioning how North Korean propagandists would depict it as Trump paying homage to 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 Kim and Moon will hold their talks; the US 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, Ulaanbaatar, as possible venues.

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 US 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, US 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, such as Trump’s new national security adviser, John 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 Trump’s term ends in January 2021, according to scholars advising Moon’s government. They said that a key challenge for Moon would be to persuade Kim and Trump to exchange key trust-building steps soon after they meet, such as granting inspectors unfettered access to the North’s nuclear facilities.