U.S. officials believe North Korea has increased fuel production for nuclear missiles at several secret research facilities and that Kim Jong Un may plan to hide the sites amid negotiations over the country's nuclear program, according to a report Friday.

Five U.S. officials familiar with an unreleased U.S. intelligence assessment told NBC News that North Korean forces have stepped up production of necessary fuel and components for nuclear weapons in recent months as both sides engaged in diplomacy.

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President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE and Kim signed an agreement at a historic meeting in Singapore earlier this month committing the U.S. to unspecific security guarantees for North Korea in exchange for Pyongyang agreeing to move toward denuclearization.

"There's no evidence that they are decreasing stockpiles, or that they have stopped their production," one unnamed U.S. official briefed on the latest intelligence told NBC. "There is absolutely unequivocal evidence that they are trying to deceive the U.S."

Five officials told NBC that North Korea's military had stepped up production of weapons-grade enriched uranium in recent months even as Kim met with Trump and the two leaders agreed to work toward the complete denuclearization of the Korean Peninsula.

The same number of intelligence officials said in the report that it was likely that North Korea's leader was attempting to deceive Trump and the U.S. while clinging to his country's controversial weapons program.

"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.

However, one senior official told NBC that the administration was aware of Kim's efforts and was watching North Korea "closely" to ensure no part of the country's weapons program escaped U.S. notice.

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

The assessment from U.S. intelligence officials comes in apparent contrast to comments from the president, who has declared North Korea's nuclear threat to be over amid his negotiations with the country.

"There is no longer a Nuclear Threat from North Korea. Meeting with Kim Jong Un was an interesting and very positive experience. North Korea has great potential for the future!" Trump tweeted just weeks ago after his summit with Kim.

Trump shocked many in Washington and around the world earlier this year when he became the first U.S. president since the Korean War to accept a meeting with a North Korean leader.

The move followed months of eased tensions between North and South Korea as a result of increased cooperation around the 2018 Winter Olympic Games, held in South Korea.