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.
“I would be surprised in a negative way if he did anything that was not per our understanding. But we’ll see what happens,” he said.
“I would be very disappointed if I saw 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.
The BBC’s Seoul correspondent Laura Bicker says that North Korea might be testing the US after talks in Hanoi between Donald Trump and Kim Jong-un broke down, hoping that the US will offer a better deal to avoid a launch.
He also adds that experts say the rockets used to launch satellites are usually unsuitable for use as long-range missiles.