"I told President Trump that at our meeting North Korean leader Kim Jong Un said he's committed to denuclearization," said South Korean National Security Advisor Chung Eui-yong, who came to Washington after talks in Pyongyang.

"Kim pledged that North Korea will refrain from any further nuclear or missile tests," he told reporters, in an extraordinary appearance on the White House driveway.

"He understands that the routine joint military exercise between the Republic of Korea and the United States must continue and he expressed his eagerness to meet president Trump as soon as possible," he said.

"President Trump appreciated the briefing, and said he would meet Kim Jong Un by May to achieve permanent denuclearization," he added. In Seoul, South Korean officials told reporters that the talks would take place "by the end of May."

Trump's spokeswoman Sarah Sanders confirmed that the US leader "will accept the invitation to meet with Kim Jong Un at a place and time to be determined."

"We look forward to the denuclearization of North Korea. In the meantime, all sanctions and maximum pressure must remain," she said.

Trump did not appear alongside Chung, but earlier had made a surprise appearance in the White House press briefing room to promise that South Korea would soon make a "major announcement".

South Korea announced on Tuesday that the North had promised there would be "no reason" to hold on to its nuclear weapons "if military threats towards the North are cleared and the security of its regime is guaranteed."

Trump welcomed this apparent offer as "very positive," implying that he believes he is on the verge of a swift and unexpected breakthrough for his so-called "maximum pressure campaign" to force Kim to accept disarmament talks.

North Korea has been placed under a stern international sanctions regime, backed by UN Security Council resolutions and importantly by key power China, the isolated regime's only ally and significant trading partner.

Earlier Thursday, China's Foreign Minister Wang Yi had also cautiously welcomed the apparent breakthrough, and called for Washington and the North "to engage in dialogue sooner rather than later."

Washington has long insisted that it will not allow Pyongyang to continue building and testing a nuclear arsenal that Kim's regime now boasts is capable of delivering thermonuclear warheads to US cities.

But as late as Thursday morning, officials were warning that negotiations might still be far off.