New satellite imagery taken Tuesday shows a gathering of up to 100 people at the administrative area of North Korea's main nuclear test site — a level of activity not been seen since January 2013, a month before Pyongyang's third atomic test, the influential 38 North website said Wednesday.

The large gathering of people apparently standing in formation or watching an event at the Punggye-ri nuclear test site is the latest development in a series of recent signs that the North may be readying to conduct its sixth nuclear test, said 38 North, a project of the U.S.-Korea Institute at Johns Hopkins University.

In a report a day earlier analyzing satellite images taken Saturday, the website said there were strong suggestions that preparations for a nuclear test are entering their final stages. These included the presence of vehicles and the apparent laying of communications cables at the entrance to an underground test tunnel. The report said water was also being pumped out of the tunnel or being drained downhill, presumably to keep it dry for monitoring or communications equipment.