The warning came amid concerns over the North's satellite rocket launch site, where some rebuilding activity has been observed in recent weeks, triggering international alarm that Pyongyang might be preparing a long-range missile or space launch.

"We have no intention to yield to the US demands in any form, nor are we willing to engage in negotiations of this kind," the Russian news agency TASS cited vice foreign minister Choe Son Hui as saying.

Kim would soon make an official statement on the actions his country would take, Choe told reporters and foreign diplomats in Pyongyang.

Trump told a post-summit press conference in Hanoi that the North Korean leader had promised he would maintain his moratorium on missile and nuclear tests.

Any launch would send the denuclearisation talks into complete disarray, after they were left stuttering when the summit ended without agreement two weeks ago.

Choe -- who was present in Vietnam -- blamed the US for the failure, saying Secretary of State Michael Pompeo and National Security Adviser John Bolton "created the atmosphere of hostility and mistrust" and "obstructed" Kim and Trump's "constructive effort".

"As a result, the summit ended with no significant result," she added.

It is a change of tone from Pyongyang, after both sides expressed their willingness to carry on the discussion process in Hanoi.

Ankit Panda, of the Federation of American Scientists, said Kim's unilateral declaration of the ICBM testing moratorium was "now in question".

It could mean a decision had already been made, he tweeted, but added that it did not mean a launch was imminent.

"One play is for Kim to reaffirm the moratorium. Kim looks big," he said. "That also keeps China happy for now.

"Another play is to renounce the moratorium, which would be risky," as it would upset China, be "disappointing" to Trump, and "squanders the inter-Korean process".

- 'Big deal' -

Seoul's presidential office sought to play down the vice-minister's comments, saying it was "premature to assess the current situation only with Choe's remarks".

South Korea will continue to work for the resumption of talks, it added.

Washington wants what administration officials have called a "big deal", with the complete elimination of weapons of mass destruction in return for the dropping of sanctions that have hit the isolated North's economy.

"Nobody in the administration advocates a step-by-step approach," a senior State Department official told reporters last week.

The North says it only called for the partial lifting of UN sanctions imposed 2016-17 that affect people's livelihoods.

But Washington sees these measures as the bulk of economic sanctions that brought Kim to the table, and believes without them it would lose leverage in future talks.

Pyongyang favours a more incremental approach, with Kim proposing dismantling facilities at its Yongbyon nuclear complex in exchange for lifting the main sanctions -- an offer Trump refused in Hanoi despite the vaunted "chemistry" between the pair.

In his New Year's speech -- a key political event in the North -- Kim said he would be "compelled to find a new way for defending the sovereignty" of the state if Washington "persists in imposing sanctions and pressure."