Is North Korea still secretly building nuclear weapons?

According to reports, United States officials familiar with the intelligence confirmed North Korea continues to produce new missiles at a known manufacturing site that developed the country's first intercontinental-range ballistic missiles (ICBMs), which are capable of reaching the U.S.

The report came weeks after President Donald Trump tweeted there was no longer a nuclear threat from North Korea after his historic meet with the country's leader Kim Jong Un.

Officials told NBC News that North Korea has also increased its production of fuel for nuclear weapons at multiple secret sites in the recent months, and added Kim might try to hide the existence of these facilities as he seeks more concessions in nuclear talks with the Trump administration.

The news, which was first reported by the Washington Post, was later confirmed by the Diplomat as well.

"Work on the new missile likely began after the summit," an official said, referring to the iconic meet between Trump and Kim in Singapore on July 12.

North Korea had vaguely agreed to end its extreme hostile relations with the U.S. and start a step-by-step and simultaneous denuclearization process of the Korean Peninsula in the historic meet.

Meanwhile, the Washington Post claimed in a recent report that senior North Korean officials are planning to deceive U.S. about the number of nuclear warheads, missiles they have, and the types and numbers of facilities as well. According to the report, U.S. agencies have gathered information that their strategy is to potentially assert denuclearization by declaring and disposing of 20 warheads, while retaining dozens more.

"We see them going to work, just as before," a U.S. official told the Washington Post.

The findings, which according to officials include satellite photos taken in recent weeks, show that there was ongoing activity within nuclear and missile facilities in North Korea. The recent reports were released after news of a suspected uranium-enrichment facility — called Kangson, which was being operated by North Korea in secret — surfaced.

At least one or two Hwasong-15/KN22 ICBMs were being manufactured at the site known as the Sanum-dong Research Center, which is situated outside the North Korean capital of Pyongyang, the Diplomat reported.

Last week, Secretary of State Mike Pompeo acknowledged during a Senate testimony that North Korean factories continue to produce fissile material used in making nuclear weapons, but declined to say whether Pyongyang was building new missiles. He also assured disarmament talks with North Korea remained on track.

"We have not been taken for a ride," he said.

Independent missile experts have reported observing activity consistent with missile construction at the Sanumdong plant as well.

"It's active. We see shipping containers and vehicles coming and going. This is a facility where they build ICBMs and space-launch vehicles," Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies, said.

Earlier on June 26, 38North, a website devoted to analysis of events in and around North Korea, released new satellite images of what seemed to be infrastructure improvements being made at North Korea's Yongbyon Nuclear Research Facility.

The continued activity inside North Korea's weapons complex was not surprising, several U.S. officials and private analysts told the Washington Post, since Kim made no public promise at the summit to halt work at nuclear and missile facilities in North Korea.

Ken Gause, a North Korea expert at the Center for Naval Analysis, said the North Koreans never agreed to give up their nuclear program.

"Regime survival and perpetuation of Kim family rule are Kim's guiding principles," he said. "The nuclear program provides them with a deterrent, in their mind, against regime change by the United States. Giving up the nuclear capability will violate the two fundamental centers of gravity in the North Korean regime."

A South Korean official told a BBC correspondent they are "closely monitoring the continuing cooperation between North Korea and the US, but it is difficult to comment on intelligence issues."