WASHINGTON — U.S. intelligence agencies believe that North Korea has increased its production of fuel for nuclear weapons at multiple secret sites in recent months — and that Kim Jong Un may try to hide those facilities as he seeks more concessions in nuclear talks with the Trump administration, U.S. officials told NBC News.

The intelligence assessment, which has not previously been reported, seems to counter the sentiments expressed by President Donald Trump, who tweeted after his historic June 12 summit with Kim that "there was no longer a nuclear threat from North Korea."

Analysts at the CIA and other intelligence agencies don't see it that way, according to more than a dozen American officials who are familiar with their assessments and spoke on the condition of anonymity. They see a regime positioning itself to extract every concession it can from the Trump administration — while clinging to nuclear weapons it believes are essential to survival.

The White House did not immediately respond to a request for comment.

In recent months, even as the two sides engaged in diplomacy, North Korea was stepping up its production of enriched uranium for nuclear weapons, five U.S. officials say, citing the latest intelligence assessment. North Korea and the U.S. agreed at the summit to "work toward" denuclearization, but there is no specific deal. On Trump's order, the U.S. military canceled training exercises on the Korean peninsula, a major concession to Kim.

While the North Koreans have stopped missile and nuclear tests, "there's no evidence that they are decreasing stockpiles, or that they have stopped their production," said one U.S. official briefed on the latest intelligence. "There is absolutely unequivocal evidence that they are trying to deceive the U.S."

Four other officials familiar with the intelligence assessment also said North Korea intended to deceive the U.S.

View this graphic on nbcnews.com

U.S. intelligence agencies have stepped up their collection against North Korea in recent years, and it appears to be paying off with greater insights into a country that has long been the world's hardest spying target, officials say. NBC News agreed to withhold some details of the latest intelligence assessment that officials said could put sources at risk.

"There are lots of things that we know that North Korea has tried to hide from us for a long time," a U.S. intelligence official said.

It's long been understood that North Korea had at least one undeclared facility to enrich nuclear fuel, aside from Yongbyon, its main nuclear site.

"When North Korea constructed the enrichment facility at Yongbyon in 2009, the North Koreans did so at a pace that suggested this was not their first rodeo, i.e. not the first time they had assembled large cascades of centrifuges," said Jeffrey Lewis, director of the East Asia Nonproliferation Program for the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies at Monterey.

A satellite image of North Korea's Yongbyon facility annotated by 38north.org. Airbus Defense and Space and 38 North

Joel Wit, who negotiated a 1994 nuclear agreement with North Korea, said the U.S. always believed North Korea had two facilities to enrich nuclear material: Yongbyon and a second site the U.S. is aware of but whose name has not been disclosed.

"People have been open to the possibility there might be more," he said.

The latest U.S. intelligence assessment concludes that there is more than one secret site, officials tell NBC News. The question is whether Kim will be willing to admit it.

"This is why people want North Korea to declare all its facilities up front," said Wit, a former Clinton administration official and senior fellow at the Stimson Center who founded a website devoted to North Korea, 38north.org.

The intelligence assessment comes on the heels of a report by 38north.org showing that North Korea was continuing to make improvements at its major disclosed nuclear facility at Yongbyon.

"The observed activity appears inconsistent with a North Korean intent to abandon its nuclear weapons programs," said Bruce Klingner, a former CIA analyst and North Korea expert at the Heritage Foundation. "There seems little reason to continue expansion plans if the regime intended to dismantle them as would be required under a denuclearization agreement."

One senior U.S. intelligence official offered a different view, noting that the decision by Kim to suspend nuclear and missile tests was unexpected, and the fact that the two sides are talking is a positive step.

But that official also acknowledged that intelligence analysts expect the Kim regime to try to deceive the United States.

"Work is ongoing to deceive us on the number of facilities, the number of weapons, the number of missiles," he said. "We are watching closely."