Satellite images of a facility near Pyongyang suggest that North Korea may be preparing to launch a missile or a satellite.
The increase in activity is around a site known as Sanumdong, where North Korea assembled most of its ballistic missiles and rockets.
It comes after reports earlier this week that North Korea's main rocket launch site at Sohae had been rebuilt.
Work to dismantle Sohae began last year but stopped as US talks stalled.
On Friday US President Donald Trump said he would be disappointed if North Korea was to resume weapons testing.
Analysts believe that it is more likely at this stage that Pyongyang is preparing to launch a satellite rather than test a missile.
However the US said earlier this week that this would still be inconsistent with the commitments North Korean leader Kim Jong-un has made to President Trump.
What is happening at Sanumdong?
Large vehicles have been seen moving around Sanumdong, activity which has in the past indicated that North Korea was at least preparing to move some kind of missile or rocket to a launch area.
The satellite images were published by the US public radio network NPR.