US president Donald Trump and North Korea’s Kim Jong Un have come together for a momentous summit that seemed unthinkable just months ago.

The leaders met at a Singapore island resort for a meeting that could determine historic peace or raise the spectre of a growing nuclear threat, with Mr Trump pledging that “working together we will get it taken care of”.

After shaking hands warmly in front of a row of alternating US and North Korean flags, they then moved into a roughly 40-minute one-on-one meeting, joined only by their interpreters, before including their advisers.

For all the upbeat talk, it remained to be seen what, if any, concrete results the sit-down would produce.

“We are going to have a great discussion and I think tremendous success. We will be tremendously successful,” Mr Trump said before their private session.

Mr Kim said through an interpreter: “It wasn’t easy for us to come here. There was a past that grabbed our ankles and wrong prejudices and practices that at times covered our eyes and ears. We overcame all that and we are here now.”

Aware that the eyes of the world were on a moment that many people never expected to ever see, Mr Kim remarked that many of those watching “will think of this as a scene from a fantasy… science fiction movie.”

In the run-up to the meeting, Mr Trump had predicted the two men might strike a nuclear deal or forge a formal end to the Korean War in the course of a single meeting or over several days.

But on the eve of the summit, the White House unexpectedly announced Mr Trump would depart Singapore by Tuesday evening, raising questions about whether his aspirations for an ambitious outcome had been scaled back.

Trump Kim Summit Donald Trump meets with Kim Jong Un on Sentosa Island (Evan Vucci/AP)

Critics of the summit leapt at the handshake and the moonlight stroll Mr Kim took on Monday night along the glittering Singapore waterfront, saying it was further evidence that Mr Trump was helping legitimise Mr Kim on the world stage as an equal of the US president.

Mr Kim has been accused of horrific rights abuses against his people. During his stroll, crowds yelled out Mr Kim’s name and jostled to take pictures, and the North Korean leader posed for a selfie with Singapore officials.

Mr Trump responded to that criticism on Tuesday on Twitter, saying: “The fact that I am having a meeting is a major loss for the US, say the haters & losers.”

But he added “our hostages” are back home and testing, research and launches have stopped.

Mr Trump also tweeted: “Meetings between staffs and representatives are going well and quickly … but in the end, that doesn’t matter. We will all know soon whether or not a real deal, unlike those of the past, can happen!”

Addressing reporters on Monday, US Secretary of State Mike Pompeo sought to keep expectations in check, saying: “We are hopeful this summit will have set the conditions for future successful talks.”

The summit capped a dizzying few days of foreign policy activity for Mr Trump, who shocked US allies over the weekend by using a meeting in Canada of the G7 to alienate America’s closest friends in the West.

Lashing out over trade practices, Mr Trump lobbed insults at his G7 host, Canadian prime minister Justin Trudeau.

Mr Trump left that summit early and, as he flew to Singapore, tweeted that he was yanking the US out of the group’s traditional closing statement.

As for Singapore, the White House said Mr Trump was leaving early because negotiations had moved “more quickly than expected,” but gave no details about any possible progress in preliminary talks.

On the day before the meeting, weeks of preparation appeared to pick up in pace, with US and North Korean officials meeting throughout Monday at a Singapore hotel.

The president planned to stop in Guam and Hawaii on the way back to Washington.

Mr Trump spoke only briefly in public on Monday, forecasting a “nice” outcome.

Mr Kim spent the day mostly out of view — until he embarked on the late-night sightseeing tour of Singapore, including the Flower Dome at Gardens by the Bay, billed as the world’s biggest glass greenhouse.

As Mr Trump and Singapore’s prime minister Lee Hsien Loong sat down for a working lunch at the Istana house, the president sounded optimistic, telling Lee, “We’ve got a very interesting meeting in particular tomorrow, and I think things can work out very nicely”.

It was a striking about-face from less than a year ago, when Mr Trump was threatening “fire and fury” against Mr Kim, who in turn scorned the American president as a “mentally deranged US dotard.” As it happens, the North Korean and the American share a tendency to act unpredictably on the world stage.

Beyond the impact on both leaders’ political fortunes, the summit could shape the fate of countless people — the citizens of impoverished North Korea, the tens of millions living in the shadow of the North’s nuclear threat, and millions more worldwide.

Or, it could amount to little more than a much-photographed handshake.