US officials crossed into North Korea yesterday to hold talks on preparations for a possible summit, a U.S. newspaper reported, as North Korean leader Kim Jong Un reaffirmed his commitment to meet with U.S. President Donald Trump.

Both Pyongyang and Washington are pressing ahead on plans for a meeting between the two leaders after Trump pulled out of the scheduled June 12 summit on Thursday, only to reconsider the decision the next day.

The Washington Post, citing a person familiar with the arrangements, said Sung Kim, a former US ambassador to South Korea and former nuclear negotiator with the North, was leading the preparations on the US side.

He crossed into North Korean territory with Allison Hooker, the Korea expert on the White House National Security Council. They met with Choe Son Hui, the North Korean vice foreign minister, the Post said. Pentagon official Randall Schriver is also in Seoul currently, the Post said.

The meetings are expected to continue on Monday and Tuesday and are focused on the issue of North Korea's nuclear weapons program, it said.

South Korean President Moon Jae-in said earlier that he and North Korea's Kim agreed at a surprise meeting on Saturday that the possible North Korea-US summit must be held, Moon told a news conference in Seoul.

Kim reaffirmed his commitment to "complete" denuclearisation of the Korean Peninsula and to a planned meeting with Trump, Moon told a news conference in Seoul.

"Chairman Kim and I have agreed that the June 12 summit should be held successfully, and that our quest for the Korean peninsula's denuclearisation and a perpetual peace regime should not be halted," Moon said.

The meeting was the latest dramatic turn in a week of diplomatic ups and downs surrounding the prospects for an unprecedented summit between the United States and North Korea, and the strongest sign yet that the leaders of the two Koreas are trying to keep the on-again off-again meeting on track.