US officials familiar with intelligence assessments were reported to have told the NBC broadcaster that Pyongyang appeared to be planning to deceive the US administration to elicit more concessions.
But the US Central Intelligence Agency, which has reportedly stepped up spying efforts in North Korea, alleges that production of weapons-grade enriched uranium has been increasing.
North Korea has only one declared enrichment facility at Yongbyon, according to NBC, but sources told the broadcaster there seemed to be more than one unreported site.
Trump said after his June 12 summit with North Korean leader Kim Jong-un there was no longer a nuclear threat from Pyongyang. Kim said in a joint declaration he was committed to full denuclearization, a prerequisite for easing sanctions on his country.
Earlier, 38 North, a project by the Henry L Stimson Center, reported that construction work on nuclear facilities at the North Korean Nyongbon Nuclear Scientific Research Center had not abated and was progressing at a "rapid pace."