Unnamed US officials told the Washington Post that spy satellites had spotted continuing activity at a site that has produced ballistic missiles.
Reuters quotes an official as saying it is unclear how far the work has gone.
Donald Trump met North Korea's Kim Jong-un in Singapore in June.
After the first meeting between sitting leaders from the two countries, the two men pledged to work towards denuclearisation. Mr Trump later said North Korea was "no longer a nuclear threat".
But Mr Trump was criticised at home for making concessions without securing any firm commitment from Mr Kim to end the nuclear and missile programmes.
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.
What do the latest reports say?
On Monday, the Washington Post newspaper quoted officials as saying North Korea appeared to be building one or two new liquid-fuelled intercontinental ballistic missiles (ICBMs) at the Sanumdong facility near the capital, Pyongyang.
The factory is known to have produced the Hwasong-15, the first North Korean ICBM capable of reaching the US.
However, a US official told news agency Reuters that a liquid-fuelled ICBM didn't "pose nearly the threat that a solid-fuelled one would because they take so long to fuel".
Reuters also added that satellite imaging showed vehicles moving in and out of the facility, but not the extent of any missile construction.
What are experts saying about this?
Satellite imagery of the Sanumdong facility shows that the site is "active", Jeffrey Lewis, a nuclear expert at the Middlebury Institute of International Studies (MIIS) told the Washington Post.
"[The facility] is not dead, by any stretch of the imagination," said Mr Lewis. "We see shipping containers and vehicles coming and going. This is a facility where they build ICBMs and space-launch vehicles."
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