Donald Trump stepped onto North Korean soil in a historic first Sunday as he met Pyongyang's leader Kim Jong Un in a moment of high diplomatic drama on the world's last Cold War frontier.

Moments after becoming the only sitting US president to set foot inside North Korea, Trump brought Kim back over the dividing line for a meeting where they agreed to start working-level talks on Pyongyang's nuclear weapons.

Trump, 73, also said he had invited 35-year-old Kim to the White House "anytime he wants to do it".

Donald Trump stepped onto North Korean soil in a historic first Sunday as he met Pyongyang's leader Kim Jong Un in a moment of high diplomatic drama on the world's last Cold War frontier.

Moments after becoming the only sitting US president to set foot inside North Korea, Trump brought Kim back over the dividing line for a meeting where they agreed to start working-level talks on Pyongyang's nuclear weapons.

Trump, 73, also said he had invited 35-year-old Kim to the White House "anytime he wants to do it".

Contact between the two sides has since been minimal -- with Pyongyang issuing frequent criticisms of the US position -- but the two leaders exchanged a series of letters before Trump issued his offer to meet at the DMZ.

Trump's entry onto North Korean soil -- which he said was uncertain until the last moment -- is an extraordinary sequel to the scene at Kim's first summit with Moon Jae-in last year, when the young leader invited the South Korean president to walk over the Military Demarcation Line, as the border is officially known.

A Kim trip to Washington would be even more spectacular, although Trump said the visit would happen "at the right time".

- 'Very legendary' -

Moon seized on last year's Winter Olympics to broker the process between Pyongyang and Washington, after tensions soared in 2017 when the North carried out multiple missile launches and its biggest nuclear test to date, while Trump and Kim traded mutual insults and threats of war.

The significance of the meeting in the no-man's-land splitting the peninsula -- where the two sides and their allies fought each other to a standstill in the 1950-53 Korean War -- was "obvious", said Stimson Centre Asia analyst David Kim.

"It's historic for Trump to be the first US President (to) enter North Korea soil, historic for Moon to meet, albeit briefly, with both leaders."

The meeting had the "potential to kick-start stalled negotiations", he told AFP, but added that working-level discussions would be crucial.

"What we need is substance, not theatrics."

US Secretary of State Mike Pompeo told reporters the two sides would "start exchanging ideas" probably around mid-July.

In Washington, Democratic White House contenders guardedly welcomed the meeting of the two leaders though some questioned the wisdom of granting Kim "legitimacy" despite the lack of progress on curbing North Korea's nuclear arsenal.

"I have no problem with him sitting down with Kim Jong Un in North Korea or any place else. I don't want it to be a simple photo opportunity," said Senator Bernie Sanders.

The North has carried out six nuclear tests and developed missiles capable of reaching the entire US mainland.

"What's happened here is that this president has raised the profile of a dictator like Kim Jong Un," while "we haven't gotten anything out of it," another Democratic White House hopeful, Julian Castro, said on ABC.

The DMZ has been a regular stop for US presidents visiting the South, a security ally, while Panmunjom saw the first two summits between Moon and Kim last year.

"This was a great day," the US president said before flying out on Air Force One.

"This was a very legendary, very historic day."