Russian Foreign Minister Sergey Lavrov is in Pyongyang on Thursday, meeting with North Korean leader Kim Jong Un to discuss the state of talks with South Korea - and inviting Kim to visit President Vladimir Putin in Russia.
After decades of tension on the Korean peninsula and antagonism with the United States, Washington has been demanding North Korea abandon its nuclear weapons program amid reports that it is close to being able to launch a nuclear-tipped missile capable of reaching the USA mainland.
Even as he pulled out, though, Trump urged Kim to "call me or write" if he wanted to revive the meeting.
"We'll be meeting in June 12 in Singapore", Trump said, after Kim Yong Chol drove away.
But analysts say North Korea will be unwilling to cede its nuclear deterrent unless it is given security guarantees that the U.S. will not try to topple the regime.
This Reuters photo shows Kim Yong-chol, a vice chairman of North Korea's ruling Workers' Party's Central Committee, arriving at his hotel in NY on May 30, 2018.
He would not say that the summit is a definite go for Singapore on June 12 and could not say if that decision would be made after Trump reads Kim Jong-un's letter.
Speaking after the Oval Office meeting, the president said he looks forward to the day when he can "take the sanctions off" North Korea.
The Oval Office talks and letter delivery came only a week after Trump threatened to consign the entire process to history, abruptly cancelling the summit in a sharply worded letter, only to revive preparations shortly afterwards. But he threatened a flurry of new sanctions on the rogue regime if "talks break down".
"We'll be meeting on June 12 in Singapore".
Kim Yong-chol, vice chairman of the North Korean ruling party's central committee, was allowed into the United States despite being on a US sanctions list, and North Korean officials are not normally allowed to travel outside the NY area. North Korea's conciliatory response to that letter appears to have put the summit back on track.
Both sides have committed themselves to the "denuclearisation" of the Korean peninsula, but it is far from clear if Trump's mission to secure Pyongyang's complete disarmament can be aligned with Kim's quest to win global respect and protection.
In Seoul, US negotiators on Friday expressed optimism after meeting their North Korean counterparts for preparatory talks at Panmunjom, on the fortified border between the two Koreas.
He tweeted from New York: "Good progress today during our meetings" with Kim and his team. He also softened his tone, saying, "I don't want to use the term maximum pressure anymore".
Most expert observers are sceptical that even an unprecedented summit between the two leaders can lead to a rapid breakthrough, and Trump admitted it would be a long and hard process.
Lavrov was the first Russian official to meet with Kim this year.