Donald Trump and Kim Jong Un pose before a meeting at the Sofitel Legend Metropole hotel in Hanoi. File / Saul Loeb / AFP

US President Donald Trump on Saturday invited North Korean leader Kim Jong Un to meet for a historic handshake at the demilitarised zone (DMZ) that divides the Korean peninsula, and said he would have "no problem" stepping over the border.

The invitation issued on Twitter caught observers by surprise.

If Kim accepts, it would be the third meeting between the leaders of the two wartime enemies amid efforts to contain North Korea's nuclear ambitions.

The North's official KCNA news agency quoted Vice Foreign Minister Choe Son Hui as saying the offer was "a very interesting suggestion" but that no official request had been received.

Trump lobbed the shock invitation on Twitter from the G20 summit in Osaka, saying: "if Chairman Kim of North Korea sees this, I would meet him at the Border/DMZ just to shake his hand and say Hello(?)!"

He later said he would have "no problem" stepping into the North with Kim − in what would be a dramatic gesture re-enacting an extraordinary 2018 scene when the young leader invited South Korean President Moon Jae-in to walk over the Military Demarcation Line that forms the border between the Koreas.

"Sure I would, I would. I'd feel very comfortable doing that. I'd have no problem," Trump told reporters.

The US leader said the invitation was spontaneous, but it comes amid a recent flurry of diplomacy over North Korea's nuclear programme after a Trump-Kim summit in Hanoi collapsed without an agreement.

Speculation grew that something was afoot when US Secretary of State Mike Pompeo skipped a G20 dinner on Friday without giving a reason.

"We'll see. If he is there, we will see each other for two minutes," said Trump.

"We seem to get along... That's a good thing, not a bad thing," he added.

North Korea's Choe said if a meeting took place it would be "another meaningful occasion in further deepening the personal relations between the two leaders and advancing the bilateral relations."

The North's speedy response to Trump's offer, and from such a senior figure, was seen by analysts as an indication that Kim would agree to the meeting as soon as Washington followed up with an official proposal.

Cheong Seong-chang, a senior researcher at the private Sejong Institute in Seoul, said Kim had "practically accepted" Trump's invitation.

"If he (Kim) isn't interested he would not release such a statement to begin with."

Twitter is not available to ordinary North Korean citizens, who have no access to the global internet, but Trump told reporters that Kim "follows" his account and that a "quick" response had been received to his offer.

Later, at a dinner with the South's President Moon Jae-in after he flew to Seoul, he was asked about the meeting and said: "We're gonna see. They're working things out right now."

But there was scepticism that a fleeting frontier photo opportunity, no matter how symbolic, would help to bridge the two sides' differences over denuclearisation.

"For talks to have real legs, either Kim must credibly commit to denuclearisation or Trump must credibly agree to allow Kim to keep some of his nuclear weapons," said Scott Seaman, director of Eurasia Group Asia.

"Without a shared end goal, creating a viable roadmap to reaching it will remain impossible."

Agence France-Presse