US president Donald Trump and North Korea’s Kim Jong Un have kicked off a momentous summit, with Mr Trump declaring the two would have a “great discussion” and Mr Kim saying they had overcome “obstacles” to get to this point.

Before a row of alternating US and North Korean flags, the leaders shook hands warmly at a Singapore island resort, creating an indelible image of two unorthodox leaders as they opened a conversation that could determine historic peace or raise the spectre of a growing nuclear threat.

Mr Trump and Mr Kim planned to meet one on one for most of an hour – joined only by interpreters. Then aides to each were to join for more discussions and a working lunch.

But even before they met, Mr Trump announced plans to leave early, raising questions about whether his aspirations for an ambitious outcome had been scaled back.

The first meeting of a sitting US president and a North Korean leader was the product of dizzying weeks of negotiations over logistics and policy.

Up early in Singapore, Mr Trump tweeted with cautious optimism: “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!”

In the run-up to the talks, Mr Trump had hopefully 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, meaning his time with Mr Kim would be fairly brief. And Secretary of State Mike Pompeo sought to keep expectations for the summit in check.

“We are hopeful this summit will have set the conditions for future successful talks,” Mr Pompeo said, describing a far more modest goal than Mr Trump had outlined days earlier.

Trump Kim SummitDonald Trump gives North Korea leader Kim Jong Un a thumbs up at their meeting at the Capella resort on Sentosa Island (Evan Vucci/AP)

The sudden change in schedule added to a dizzying few days of foreign policy activity for Mr Trump, who shocked US allies over the weekend when he used a meeting of the G7 in Canada 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 the summit early, and as he flew to Singapore, he tweeted that he was yanking the US out of the group’s traditional closing statement.

Singapore summit graphic(PA Graphics)

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 his 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 left his hotel for a late-night tour of Singapore sights, including the Flower Dome at Gardens by the Bay, billed as the world’s biggest glass greenhouse.

South Korea Trump Kim SummitPeople celebrate as they watch a TV showing Donald Trump meeting with Kim Jong Un (Ahn Young-joon/AP)

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.” Mr Trump had earlier tweeted about “excitement in the air!”

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.