MOON TOWNSHIP, PENNSYLVANIA (REUTERS, AFP) - US President Donald Trump said on Saturday (March 10) his planned meeting with North Korean leader Kim Jong Un could fizzle without an agreement or it could result in "the greatest deal for the world" to ease nuclear tensions between the two countries.

"I may leave fast" if progress does not seem possible, Trump said at a campaign rally for Republican congressional candidate Rick Saccone in western Pennsylvania.

Trump said he believes North Korea wants to make peace and that, "I think it's time."

A time and place to meet has not yet been set, although the meeting is supposed to happen by the end of May.

"Who knows what's going to happen?" said Trump, who added that if the meeting takes place,

"I may leave fast or we may sit down and make the greatest deal for the world."

Trump made the shocking decision on Thursday to meet with Kim after the North Korean leader's invitation was relayed by a South Korean delegation who visited the White House. The move abruptly reversed decades of US policy aimed at preventing North Korea from developing nuclear weapons and ballistic missiles.

"I think they want to make peace. I think it's time," Trump told a crowd of supporters at the rally in Pennsylvania, adding that he believed Pyongyang when they said "they are not sending missiles up" as the two sides work out the arrangements for a historic meeting.

"And I think we've shown great strength. I think that's also important," Trump added.

He earlier said he predicted "tremendous success" in upcoming groundbreaking talks with Kim, adding that the reclusive state had promised not to shoot missiles in the interim.

The comments came after the American leader said he has received encouragement from the leaders of China and Japan as he moves toward the high stakes summit, announced suddenly this week.

"I think North Korea is going to go very well, I think we will have tremendous success... We have a lot of support," Trump told reporters before boarding his Marine One helicopter to travel to the rally in Pennsylvania.

"The promise is they wouldn't be shooting off missiles in the meantime, and they're looking to de-nuke. So that'd be great."

Trump said China's President Xi Jinping was appreciative of his decision to opt for diplomacy rather than "the ominous alternative," while Japanese Prime Minister Shinzo Abe was "very enthusiastic" about talks with North Korea.

Trump reached out to the Asian leaders in phone calls on Friday after his stunning decision to accept an invitation to meet Kim before the end of May.

The turnabout - a huge surprise after months of intensifying brinksmanship over the North's nuclear and missile programmes - caught even Trump's top aides off guard.

Just hours before Trump made his announcement Thursday, Secretary of State Rex Tillerson had said direct talks with North Korea were "a long way" off.

Tillerson, who was travelling in Africa, cancelled his scheduled programme in Kenya because he felt unwell after a "long couple days" working on North Korea and other issues, his Undersecretary for Public Diplomacy Steve Goldstein said in a statement.

Goldstein later said Tillerson was feeling better and would resume his travel schedule on Sunday.

White House officials initially waffled on the president's intentions. "We're not going to have this meeting take place until we see concrete actions that match the words and the rhetoric of North Korea," White House spokeswoman Sarah Sanders told reporters on Friday.

In a series of tweets late Friday and Saturday, a seemingly ebullient Trump emphasised the positive.

"North Korea has not conducted a Missile Test since November 28, 2017 and has promised not to do so through our meetings. I believe they will honour that commitment!", he said. Trump praised a possible future agreement with the nuclear-armed North as "very good" for the international community as a whole.

"The deal with North Korea is very much in the making and will be, if completed, a very good one for the World. Time and place to be determined," he wrote.

Not everyone was so sanguine about the prospects of a breakthrough, however, and some Democrats shuddered at the thought of such sensitive - and potentially explosive - negotiations in the hands of an impulsive, inexperienced president.

"If you want to talk to Kim Jong Un about his nuclear weapons you need experienced diplomats," Hillary Clinton, Trump's rival in the 2016 presidential elections, told Dutch tabloid Algemeen Dagblad.

The former diplomatic chief said the State Department was "being eroded," and experienced diplomats on the North Korean issue were in short supply because many have left.

"You cannot have diplomacy without diplomats," she said, adding that "the danger is not being recognised by the Trump government."

Clinton's words echo those of veteran diplomat and former US ambassador to the United Nations Bill Richardson, who warned that negotiating with North Korea was not "reality television".

"It's a real opportunity ... I worry about the president's unpreparedness and lack of discipline. But I commend him for his very bold move in accepting the invitation," Richardson told AFP on Friday.

"But this is not 'The Apprentice' or a reality TV event. It's a negotiation with an unpredictable leader who has at least 20 nuclear weapons and who threatens the United States."