The development is the latest in a series of signs that North Korea may be preparing to conduct a sixth nuclear test and comes as US lawmakers passed a set of bipartisan measures to step up pressure on the North against its development of ballistic missiles and nuclear weapons.
Imagery of the Punggye-ri site from Tuesday shows “a heightened level of activity over the past few days”, including “one vehicle and a large contingent (70-100) of people standing in formation or watching in the courtyard of the Main Administrative Area”, the US-Korea Institute at Johns Hopkins University said on its 38 North website.
“Such a gathering hasn’t been seen since January 4, 2013, which was followed by a nuclear test on February 12,” it said.
Citing earlier imagery of the site, the website showed other signs of a possible nuclear test such as the laying of communications cables likely to be used for initiating a blast and collecting data from an explosion, as well as the pumping of water out of a test tunnel.
The institute said North Korea knows when commercial satellites are passing overhead and usually tries to avoid activity during that time.