North Korea appears to be building new ballistic missiles despite recent warming ties with the Trump administration, media reports say.
Unnamed US officials told the Washington Post that spy satellites had spotted continuing activity at a site that has produced ballistic missiles.
Reuters news agency quotes an official as saying it is unclear how far the work has gone.
President Donald Trump met North Korean leader Kim Jong-un 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.