Officials say USA spy agencies, citing evidence including recent satellite photos, have spotted signs that the North is building new intercontinental ballistic missiles, according to the Washington Post.
Citing unnamed officials familiar with the intelligence, the U.S. daily said that newly obtained evidence, including satellite photos taken in recent weeks, suggests that North Koreans were working on at least one and possibly two liquid-fueled ICBMs at a large research facility in Sanumdong on the outskirts of Pyongyang.
White House counselor Kellyanne Conway on Tuesday played down the significance of the Post report.
But he insisted the Trump administration was still making progress in its talks with Pyongyang.
North Korea is, however, dismantling some of its missile test sites.
"Things don't change overnight", senior White House adviser Kellyanne Conway told Fox News, adding that reports of US intelligence on continued intercontinental missile development suggests the North Korean negotiation "is a process".
Indeed, North Korean military officials did not claim that every item in each box went together, he said.
These are not the first reports that North Korea may be continuing its weapons programme, casting doubt on the real impact of the summit in Singapore.
The remains of 55 soldiers returned by North Korea last week are likely Americans, a US official said Wednesday.
The Sanumdong factory produced two Hwasong-15 ICBMs, North Korea's longest-range missiles, but the USA official noted that Pyongyang still had not tested a reliable re-entry vehicle capable of surviving a high-velocity trip through the Earth's atmosphere and delivering a nuclear warhead. Trump later suspended annual military drills with South Korea which North Korea had long called an invasion rehearsal.
While North Korea honored its promise to return the remains of fallen US soldiers during the Korean War, there have been troubling reports that Pyongyang will never truly disarm. The North Koreans collected the remains, and USA investigators will have to do the examination without the forensic information they normally would have, such as the approximate place of the burial and the conditions around it.
There is no longer a Nuclear Threat from North Korea, ' Trump said in a Twitter post after his historic summit with Kim in Singapore last month.
Independent experts came to the same conclusion.
"Regime survival and perpetuation of Kim family rule" are Kim's guiding principles, he said.
The US government has said it wants to resume field operations in North Korea to search for an estimated 5,300 Americans who "have not yet returned home".
But the actual details of the process, including how and when the North's nuclear program is to be dismantled, have yet to be worked out.
Why are U.S. remains in North Korea? "We have not been taken for a ride", he said.