Trump last week agreed to meet with Kim by the end of May to discuss Pyongyang's denuclearisation -- which it put on the table in exchange for security guarantees -- and predicted the talks would be a "tremendous success".

But the unorthodox announcement was made on the White House lawn by a South Korean envoy, with no American officials in attendance, and only confirmed by Washington afterwards.

More than 72 hours later, there has been no comment by the North, and nothing on the subject in its official media.

"It was a unilateral statement by Trump," said Koh Yu-hwan, a professor at Dongguk University. "Kim Jong Un just sent him a verbal message" through Seoul's intermediaries.

"To North Korea, it's not an official agreement," he told AFP. "Nothing's for sure yet. For them, an official announcement has to be based on a government-level agreement that includes the agenda and location."

The same applied to the North's summit with the South that Seoul announced for April, he added.

In addition, Pyongyang would not want to give the South credit for brokering the talks with Washington, he said, and would hold off on any announcement until its own direct contacts with the US.

In stark contrast, the US president has been itching to share his prospective meeting.

Trump has given himself a pat on the back for bringing the isolated regime to the negotiating table, showing confidence that the North was "looking to de-nuke" and that its leaders were ready to "make peace".

But analysts warn that Pyongyang's silence gives it "maximum optionality" on its next move.

"It's not tying its own hands or making any commitments," said Van Jackson, a defence expert at the Victoria University of Wellington.

"We should be wary about believing anything we're told in private -- whether by North or South Korea."

- Options A, B, C and D -

The stunning turnaround comes after a year of high tensions over the North's nuclear and missile programmes during which Trump and Kim traded personal insults and threats of war.

The summit -- if it materialises -- will be the first between a sitting US president and a North Korean leader and they will have to navigate through decades of mistrust.

Possible locations mooted in the media include the Demilitarized Zone (DMZ) that divides the two Koreas, the South's Jeju Island, Switzerland -- where Kim studied as a teenager -- Sweden, and even Washington DC.

But critics warn that the talks, entered into by an impulsive, inexperienced US president, carry high risks and could create more rumbles in an already volatile region.

They are scheduled to take place without the months of groundwork that usually precedes such meetings, and reports say Trump accepted Kim's summit invitation without consulting his top national security advisers.

"Diplomacy without a plan, process, or supporting technical expertise isn't diplomacy -- it's egoism," Jackson told AFP.

"Believing in diplomacy under those conditions is believing in magic," he added.

Analysts say Trump is giving Kim what Pyongyang wants most -- joint billing with the US -- without extracting meaningful concessions in return, and should allow his diplomats to prepare the ground first.

"The administration needs to catch up fast," said Joel Wit and Susan DiMaggio, two US analysts who have been involved in informal talks with North Korea.

"Knowing how they operate, the North Koreans probably have planned their strategy meticulously and looked out over months with 'Options A, B, C and D'," they wrote in Politico magazine.

Meanwhile Trump should remain "disciplined" and "patient", they added, urging him to refrain from "hot rhetoric and taunting tweets".

Others say Pyongyang may have been caught off-guard by the speed of the US president's acceptance.

"I think North Korea did not expect an answer to come this fast," Cho Sung-ryul of the South's Institute for National Security Strategy told local radio station CBS.

The risk is that a hastily-put-together meeting could end in failure and have wider, more dangerous repercussions, experts warn.

"Diplomacy by the wrong people in the wrong way with the wrong preparation will be a poison pill," said Jackson.

And he pointed out that Trump met the hawkish former UN ambassador John Bolton for an hour the previous day.

"If high-wire diplomacy fails, then it strengthens the case for war."