The US-North Korea nuclear summit in Hanoi ended abruptly without a deal today, with President Donald Trump saying he had decided to "walk" in the face of Kim Jong Un's demands to drop sanctions.

The much-anticipated second meeting between the two leaders was supposed to build on their historic first summit in Singapore, but they failed to sign a joint statement as initially scheduled and the talks ended in deadlock.

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Trump, Kim kick off 2nd summit

"Sometimes you have to walk and this was just one of those times," an unusually downbeat Trump told reporters.

"Basically they wanted the sanctions lifted in their entirety and we couldn't do that," Trump said before leaving Vietnam aboard Air Force One to head back to Washington. 

But Trump insisted he was "optimistic that the progress we made" before and during the summit left them "in position to have a really good outcome" in the future.

"I'd much rather do it right than do it fast," he added.

He noted Kim had vowed not to resume nuclear or ballistic missile testing -- something he previously identified as a yardstick for success -- and reiterated their "close relationship".

"We just like each other... there's a warmth that we have and I hope that stays, I think it will," he said.

'MAJOR FAILURE'

The outcome in Hanoi fell far short of the pre-meeting expectations and hopes, after critics said their initial historic meeting in Singapore was more style over substance.

The leaders walked away with no set plans for a third meeting, though Trump said he hopes to see Kim again soon.

Ankit Panda, from the Federation of American Scientists, warned on Twitter that the White House's expectation of further talks "does not have to be a perception shared in North Korea."

"Kim may have left irate, for all we know. He may have no intention of continuing this," added Panda.

In the original White House programme, a "Joint Agreement Signing Ceremony" had been scheduled in Hanoi and a working lunch for the leaders.

Instead both men left the summit venue without signing anything and Trump moved up his news conference by two hours.

"This is a major failure," tweeted Joe Cirincione, president of the Ploughshares Fund peace foundation, saying it showed the limit of summitry, with "not enough time or staff" to work out a deal.

Trump flew around the world for the meeting and Kim undertook a mammoth two-and-a-half-day trek through China in his olive green train, travelling 4,000 kilometres (2,500 miles).

Kim will stay on in Vietnam for a state visit, which will include a wreath-laying ceremony at the Ho Chi Minh mausoleum, before his expected departure Saturday. 

The US president, who touted his "special relationship" with Kim, frequently dangled the prospect of a brighter economic future for a nuclear-free North Korea, at one point saying there was "AWESOME" potential.

From the outset, he had appeared to downplay expectations of an immediate breakthrough in nuclear talks, saying he was in "no rush" to clinch a rapid deal and was content if a pause in missile testing continued.

But Harry Kazianis, Director of Korean Studies at the Centre for the National Interest, said that no agreement was better than a bad one.

"The challenge is North Korea's nuclear weapons are already a reality," he said. "Getting a deal that does little to nothing to remove that threat would be far worse than a flawed deal."

Soon after the meeting ended, South Korea said the outcome was "regrettable" but that meaningful progress had been made.

'ROCKET MAN'

In Singapore Kim and Trump signed a vague document in which Kim pledged to "work toward complete denuclearisation of the Korean peninsula".

Progress subsequently stalled, with the two sides disagreeing on what that means, as the North sought relief from sanctions and Washington pressed for concrete steps towards it giving up its weapons.

As in Singapore, the two men put on a show of bonhomie in Vietnam, appearing to share jokes in front of reporters.

Looking relaxed but appearing to say little, they indulged in a poolside stroll Thursday around the gardens of the luxury Metropole Hotel.

It was a far cry from the height of missile-testing tensions in 2017 when Trump slammed Kim as "rocket man" and the younger man branded the American president a "mentally deranged US dotard".

In apparently unprecedented scenes, Kim answered unscripted questions from foreign reporters, saying he would welcome the establishment of a US liaison office in Pyongyang, which would be a step on the way to diplomatic normalisation.

Before the summit, there was talk that there could be a political declaration ending the 1950-53 Korean War which finished technically with an armistice rather than a peace treaty.

There were also hopes Kim could pledge to destroy North Korea's decades-old Yongbyon nuclear complex, which has long been at the heart of Pyongyang's atomic development but remains shrouded in secrecy.

A patent distraction from the summit was a scandal back home in Washington with Trump's former personal lawyer and fixer Michael Cohen calling him a "racist" and a "conman" during a congressional hearing.

"He lied a lot," the president said simply in response today.