North Korea has increased production of fuel for nuclear weapons at a number of secret sites in recent months, according to US intelligence.
Agencies believe the rogue state may try to hide its activities while seeking concessions in nuclear talks with the United States, NBC News quoted officials as saying.
It comes just weeks after Donald Trump said North Korea was no longer a nuclear threat following his meeting with Kim Jong Un in Singapore.
The president said Mr Kim had promised the destruction of a missile engine testing site and the repatriation of the remains of US soldiers killed in the Korean war.
The NBC TV network quoted five US officials saying that in recent months North Korea had stepped up production of enriched uranium for nuclear weapons at the same time as it engaged in diplomacy with the US.
An intelligence assessment concludes that North Korea has more than one secret nuclear site in addition to its known nuclear fuel production facility at Yongbyon, the report said.
NBC quoted one official saying: “There is absolutely unequivocal evidence that they are trying to deceive the US.”
Another intelligence source told the network: “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 Program at California’s Middlebury Institute of International Studies, said it had long been understood that North Korea has at least one undeclared facility to enrich nuclear fuel as well as Yongbyon.
“This assessment says there is more than one secret site,” he said. “That means there are at least three, if not more sites.”
Mr Lewis said the report also implied that US intelligence had evidence to suggest North Korea did not intend to disclose one or more of the enrichment sites.
He added: “Together, these two things would imply that North Korea intended to disclose some sites as part of the denuclearisation process, while retaining others.”
At the summit, North Korea agreed to “work toward denuclearisation of the Korean Peninsula”.
However, the joint statement signed by Mr Kim and Mr Trump gave no details of how or when Pyongyang might surrender its nuclear weapons.
Before the summit, North Korea rejected the possibility of unilaterally abandoning its nuclear arsenal, which it said was a deterrent against US aggression.
US secretary of state Mike Pompeo said last week it was likely he would return to North Korea soon to try to thrash out commitments made at the summit.
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”.
Mr Trump said last week North Korea was destroying four of its big test sites and that a process of “total denuclearisation… has already started”.
However, just days ago respected North Korea monitor 38 North published satellite images that showed the Yongbyon Nuclear Scientifc Research Centre is continuing at a “rapid pace”.
The images from 21 June showed several new buildings, including an engineering office and a driveway to a building which houses a nuclear reactor.