NBC's report said North Korea has been secretly increasing production of enriched uranium for nuclear weapons at clandestine sites ― despite the vague promises made by leader Kim Jong Un during his June 12 summit in Singapore with President Donald Trump to "work towards" denuclearizing the Korean peninsula.

US Secretary of State Mike Pompeo and South Korean Minister of Foreign Affairs Kang Kyung-wha on Friday discussed next steps following the summit. "If they have the strategic decision already made to do that and they're cooperative, we can move very quickly".

North Korean leader Kim Jong Un has made a show of halting missile tests and destroying some test sites, but officials believe that secret sites remain in the country, where nuclear capabilities are still being developed.

"President Trump told us 'North Korea is no longer a nuclear threat, '" Murphy said.

Even if North Korea's promises were honest, it could take years of work, accompanied by an unprecedented agreement to grant access to outside inspectors, before USA officials could confidently say that the weapons threat has been neutralised.

U.S. officials believe that North Korea is working to hide some of its nuclear stockpile and key aspects of its production facilities from the United States, The Washington Post reported Sunday.

He said the administration was well aware of North Korea's track record over the decades in dragging out negotiations with the U.S.to continue weapons development.

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Bolton agreed that North Korea "certainly" has a history of using diplomacy as a cover, but said "Kim Jong Un was very emphatic several times in Singapore that he was different from prior regimes". I really believe he means it.

U.S. National security adviser John Bolton listens to question as speaks to the media after his talks with Russian President Vladimir Putin in Moscow, Russia, on June 27.

US Defense Secretary James Mattis meanwhile has reassured key East Asian allies that the US commitment to Seoul is "ironclad" - despite Trump's unilateral suspension of military exercises with South Korea and his lauding of Kim as a "talented guy".

"It would be physically possible to dismantle the bulk of North Korea's programs within a year", said Thomas Countryman, the State Department's top arms control officer under President Barack Obama.

Another added: 'There is absolutely unequivocal evidence that they are trying to deceive the United States'.

As of now, there is little proof that North Korea intends to go down that road, longtime North Korea observers say.

Appearing on CBS's Face the Nation, Bolton said he did not want to comment on the reports or on "anything related to intelligence" - NBC having quoted more than a dozen senior USA intelligence officials and the Post four. "The last best chance to avoid that war is a peaceful end to their nuclear program ... if they don't take it, and they play Trump like they've played everybody else, they're going to regret it".