North Korea has increased its production of fuel for nuclear weapons in recent months, US intelligence agencies believe, despite Donald Trump’s claims “there is no longer a nuclear threat” from the secretive state.

Pyongyang may be trying to hide its activities at multiple secret sites while seeking concessions in nuclear talks with the US, NBC News quoted US officials as saying.

It comes after Mr Trump claimed success in his unprecedented 12 June meeting with Kim Jong-un, the North Korean dictator, over denuclearisation.

In a report on Friday, NBC said what it described as the latest US intelligence assessment appeared to go counter to sentiments expressed by Mr Trump.

The network quoted five unidentified US officials as saying that in recent months North Korea had stepped up production of enriched uranium for nuclear weapons, even as it engaged in diplomacy with the US.

It cited US officials as saying that the intelligence assessment concludes that North Korea has more than one secret nuclear site in addition to its known nuclear fuel production facility at Yongbyon.

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“There is absolutely unequivocal evidence that they are trying to deceive the US,” NBC quoted one official as saying.

The CIA declined to comment on the report. The State Department said it could not confirm the report and did not comment on matters of intelligence. The White House did not respond to a request for comment.

The report raises further questions about North Korea’s readiness to enter serious negotiations about giving up a weapons programme that now threatens the US, in spite of Mr Trump’s enthusiastic portrayal of the summit outcome.

NBC quoted one senior US intelligence official as saying that North Korea’s decision ahead of the summit to suspend nuclear and missile tests was unexpected and the fact that the two sides were talking was a positive step.

However, he added: “Work is ongoing to deceive us on the number of facilities, the number of weapons, the number of missiles … We are watching closely.”

Jeffrey Lewis, director of the east Asia nonproliferation programme at California’s Middlebury Institute of International Studies, said there were two “bombshells” in the NBC report.

He said it had long been understood that North Korea had at least one undeclared facility to enrich nuclear fuel aside from Yongbyon.

“This assessment says there is more than one secret site. That means there are at least three, if not more sites,” he said.

Mr Lewis said the report also implied that US intelligence had reporting to suggest North Korea did not intend to disclose one or more of the enrichment sites.

“Together, these two things would imply that North Korea intended to disclose some sites as part of the denuclearisation process, while retaining others,” he said.

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North Korea agreed at the summit to “work towards denuclearisation of the Korean Peninsula”, but the joint statement signed by Mr Kim and Mr Trump gave no details on how or when Pyongyang might surrender its nuclear weapons.

Ahead of the summit, North Korea rejected unilaterally abandoning an arsenal it has called an essential deterrent against US aggression.

The US secretary of state, Mike Pompeo, said last week he would likely go back to North Korea before long to try to flesh out commitments made at the Trump-Kim meeting.

On Thursday, the Financial Times quoted US officials as saying that Mr Pompeo plans to travel to North Korea next week, but the State Department has declined to confirm this.

Bruce Klingner, a former CIA Korea expert now at the conservative Heritage Foundation think tank, said the NBC report showed Mr Trump’s statement that North Korea was no longer a nuclear threat was “absurd” and that detailed work on a verification regime was required.

Mr Trump said last week North Korea was blowing up four of its big test sites and that a process of “total denuclearisation… has already started”, but officials said there had been no such evidence since the summit.

This week, Washington-based North Korean monitoring project 38 North said recent satellite imagery showed North Korea had made rapid improvements to facilities at Yongbyon since 6 May, but it could not say if such work had continued after 12 June.