Trump's enthusiastic tweet came hours before a second summit in the Vietnamese capital Hanoi to build on their historic first meeting in Singapore last June.

In just a few words, Trump encapsulated his fervent belief that only his uniquely personal brand of diplomacy and business acumen can break a stand-off with the isolated, nuclear-armed state that has bedevilled US leaders ever since the end of the Korean war in 1953.

Trump, who has previously spoken of the "love" between him and the North Korean strongman, said that by abandoning nuclear weapons the country would very quickly emulate Vietnam -- a communist country once locked in conflict with the United States, but now a thriving trade partner.

"The potential is AWESOME, a great opportunity, like almost none other in history, for my friend Kim Jong Un. We will know fairly soon - Very Interesting!" Trump tweeted.

Trump has torn up the script by bringing Kim out of the cold, giving him a legitimacy once considered unthinkable in Washington, which has long seen North Korea as the world's most repressive state.

But now there is intense pressure for results.

The Singapore summit ended only with a vague statement promising to work "towards complete denuclearisation of the Korean peninsula".

Since then Washington and Pyongyang have disagreed even on what this means. And while North Korea has now gone more than a year without conducting missile and nuclear tests, it has done nothing to roll back the arsenal already built.

That leaves Trump, who claimed after Singapore that North Korea is "no longer a nuclear threat," scrambling to get something concrete out of Kim in Hanoi -- or face growing criticism that he is being played by the man about half his age.

On the defensive, Trump said in another tweet Wednesday that opponents in the Democratic Party should "stop talking about what I should do with North Korea and ask themselves instead why they didn't do 'it' during eight years of the (Barack) Obama Administration?"

- Dinner date -

The Kim-Trump relationship, bringing together two leaders fond of showmanship, has turned into the diplomatic equivalent of a Hollywood odd-couple bromance.

Before the Singapore get-together they were slinging bizarre insults -- Trump calling Kim "rocket man" and Kim calling him a "dotard" in response. With North Korea then busily testing missiles and conducting earth-shaking underground nuclear tests, analysts feared that the duo were egging each other on toward a catastrophic confrontation.

Now Trump promotes himself as someone who by daring to do things differently actually defused what he said was a near-war situation inherited on taking over the White House.

"He's never had a relationship with anybody from this country, and hasn't had lots of relationships anywhere," Trump said of Kim before leaving Washington.

The Hanoi summit is more elaborate than the brief first occasion. The White House said Trump and Kim will hold a 10-minute meeting, before going off for an approximately 20-minute session without aides, followed by dinner lasting around 90 minutes.

On Thursday they are due to meet again, although no details have been released, adding to the impression that much of the occasion is being decided at the last minute.

- How much to give? -

With Congress back home investigating Trump for alleged illegal activities and also trying to stop his controversial use of executive power to fund the construction of a US-Mexican border wall, the president would dearly like to change the news narrative by proclaiming a dramatic win.

Critics warn he is so keen to deal with Kim that he could give away too much in his desire to make headlines, endangering rock-solid US allies South Korea and Japan. In Singapore, Trump took his own generals by surprise when he announced a suspension of military exercises with the South -- something the North badly wanted.

Washington would ideally like Kim to dismantle a key nuclear facility at Yongbyon, allow in international inspectors, or even hand over a full list of all the country's nuclear assets -- something the North Koreans have categorically refused to do.

In return, Trump is believed to be considering dangling relief from tough international sanctions. Opening diplomatic liaison offices is another possible US concession.

Trump could go further by making a joint declaration that the 1950-53 Korean War is over, after hostilities ended with a ceasefire but no peace treaty.

Although that would be seemingly a mostly symbolic gesture, many analysts caution that it could open the door to a cascade of real-world consequences upsetting a region where the US and China are already in a growing struggle for influence.