New activity was detected this month at a site used by North Korea for previous nuclear tests, according to US analysts.
A greater number of trucks and excavation activity were spotted using satellite images of the Punggye-ri site.
“It could be related to anything from maintenance work to preparations for another nuclear test,” 38 North said on its website Thursday.
There has been mounting speculation of a fourth nuclear test or a satellite launch by Pyongyang to mark the upcoming 70th anniversary of the ruling Korean Workers’ Party, led by Kim Jong Un.
North Korea was “unlikely” to be planning a satellite launch on or before the anniversary on October 10, 38 North said, saying there were no signs of such preparations at the usual launch site.