PANMUNJOM: With grins and handshakes, President Donald Trump welcomed North Korea's Kim Jong Un at the heavily fortified Demilitarized Zone Sunday, seeking to revive talks on the pariah nation's nuclear program in a bid for a legacy-defining accord. Trump then became the first sitting American leader to step into North Korea.

The brief photo-op, another historic first in the yearlong rapprochement between the two technically warring nations, marks a return to face-to-face contact between the leaders since talks broke down during a summit in Vietnam in February. But it does little to erase significant doubts that remain about the future of the negotiations and the North's willingness to give up its stockpile of nuclear weapons.

Trump's brief crossing into North Korean territory marked the latest milestone in two years of roller-coaster diplomacy between the two nations, as personal taunts of "little rocket man" and threats to destroy the other have been ushered out by on-again, off-again talks, professions of love and flowery letters.

"I was proud to step over the line," Trump told Kim as they met in a building known as "Freedom House" on the South Korean side of the village. "It is a great day for the world."

Kim hailed the moment, saying of Trump, "I believe this is an expression of his willingness to eliminate all the unfortunate past and open a new future." He added that he was "surprised" when Trump invited to meet by a tweet on Saturday.

Peering into North Korea from atop Observation Post Ouellette, Trump told reporters before meeting Kim that there has been "tremendous" improvement since his first meeting with the North's leader in Singapore last year.

Trump claimed the situation used to be marked by "tremendous danger" but "after our first summit, all of the danger went away."

But North has yet to provide an accounting of its nuclear stockpile, let alone begin the process of dismantling its arsenal.

The meeting at the truce city of Panmunjom also represented a striking acknowledgement by Trump of the authoritarian Kim's legitimacy over a nation with an abysmal human rights record.

Trump's summit with Kim in Vietnam earlier this year collapsed without an agreement for denuclearizing the Korean Peninsula. He became the first sitting US president to meet with the leader of the isolated nation last year, when they signed an agreement in Singapore to bring the North toward denuclearization.

North Korea's nuclear threat has not been contained, Richard Haas, the president of the New York-based Council on Foreign Relations, tweeted Sunday. Haas added that the threat of conflict has subsided only because "the Trump administration has decided it can live (with) a (North Korean) nuclear program while it pursues the chimaera of denuclearization."

Substantive talks between the nations have largely broken down since the Vietnam summit. The North has baulked at Trump's insistence that it give up its weapons before it sees relief from crushing international sanctions. The US has said the North must submit to "complete, verifiable and irreversible denuclearization" before sanctions are lifted.

Every president since Ronald Reagan has visited the 1953 armistice line, except for George HW Bush, who visited when he was vice president. The show of bravado and support for South Korea, one of America's closest military allies, has evolved over the years to include binoculars and bomber jackets.

Trump, ever the showman, sought to one-up his predecessors with a Kim meeting.

The leaders met at a time of escalating tensions. While North Korea has not recently tested a long-range missile that could reach the US, last month it fired off a series of short-range missiles. Trump has brushed off the significance of those tests, even as his own national security adviser, John Bolton, has said they violated UN Security Council resolutions.