North Korea appears to be building new ballistic missiles despite recent warming ties with the Trump administration, media reports say.

Unnamed U.S. officials told the Washington Post that spy satellites had spotted continuing activity at a site that has produced ballistic missiles, the BBC wrote.

Reuters news agency quotes an official as saying it is unclear how far the work has gone.

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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.

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 U.S.

However, a U.S. 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.