In a report on Friday, the network said what it described as the latest US intelligence assessment appeared to go counter to sentiments expressed by President Donald Trump, who tweeted after an unprecedented June 12 summit with North Korean leader Kim Jong Un that "there is no longer a nuclear threat from North Korea."
NBC 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 United States.
The network 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.
"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 NBC report. The State Department said it could not confirm it and did not comment on matters of intelligence. The White House did not respond to a request for comment.
The NBC report raises further questions about North Korea's readiness to enter serious negotiations about giving up a weapons program that now threatens the United States, in spite of Trump's enthusiastic portrayal of the summit outcome.
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 Program 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.
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 denuclearization process, while retaining others," he said.
North Korea agreed at the summit to "work toward denuclearization of the Korean Peninsula," but the joint statement signed by Kim and 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.
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 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 Trump's statement that North Korea was no longer a nuclear threat was "absurd" and that detailed work on a verification regime was required.
Trump said last week North Korea was blowing up four of its big test sites and that a process of "total denuclearization ... 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 May 6, but it could not say if such work had continued after June 12.
