The new intelligence, described by four officials who have seen it or received briefings, is based on material gathered in the weeks since the summit. The officials insisted on anonymity to discuss sensitive assessments about a country that has long been one of the most difficult targets for spy agencies to penetrate. Some aspects of the US intelligence were reported on Friday by NBC News.

Specifically, the DIA has concluded that North Korean officials are exploring ways to deceive Washington about the number of nuclear warheads, and missiles and the types and numbers of facilities they have, believing that the US is not aware of the full range of their activities.

US intelligence agencies have for at least a year believed that the number of warheads is about 65. But North Korean officials are suggesting that they declare far fewer.

The lone weapons facility that has been acknowledged by North Korea is in Yongbyon, 96 kilometres north of Pyongyang. That site is estimated to have produced fissile material for as many as a couple of dozen warheads.

Meanwhile, the North Koreans also have operated a secret underground uranium enrichment site known as Kangson, which was first reported in May. That site is believed by most officials to have twice the enrichment capacity of Yongbyon. US intelligence agencies became aware of the nuclear facility in 2010.

In recent years, the US, through imagery and computer hacking, has improved its intelligence collection in North Korea. Officials in Pyongyang are seeking to obfuscate the true number of their weapons facilities, and US intelligence officials believe that more than just one hidden site exists.

The Office of the Director of National Intelligence declined requests for comment.

North Korea expert David Albright, a former United Nations weapons inspector and president of the Institute for Science and International Security, said the assessments come at a time when "there's a worry that the Trump administration may go soft, and accept a deal that focuses on Yongbyon and forgets about these other sites".

Secretary of State Mike Pompeo has acknowledged that it could take years to implement any agreement on eliminating North Korea's nuclear stockpile, a hard-won asset that North Korean leaders regard as a guarantor of their survival. Asked by senators last week about the status of private talks with North Korean officials, he declined to offer specifics.

"I'm not prepared to talk about the details of the discussions that are taking place," he said in testimony before the Senate Intelligence Committee. "I think it would be inappropriate and, frankly, counterproductive to achieving the end state that we're hoping to achieve."

Asked about Trump's claim that the North Korea threat had been eliminated, Pompeo said Trump had meant to say only that the threat had been reduced. "I don't think there's any doubt about that," he said.

While North Korea made a public show in June of demolishing the country's main nuclear weapons test site, there has been little public evidence of efforts to dismantle scores of other sites linked to production of nuclear and chemical weapons and delivery systems.

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

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

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"North Korea has made no new commitments to denuclearisation, and in fact has backed away from its previous commitments," Abraham Denmark, Asia Program director of the Woodrow Wilson International Centre for Scholars, told a House committee in late June.

"North Korea remains free to manufacture more nuclear weapons, ballistic missiles and other weapons of mass destruction - even though it has unilaterally frozen testing of its nuclear weapons and certain ballistic missiles," he said. "There is no deadline for them to eliminate their illegal capabilities, or even freeze their continued production."