"The rest of the Punggye-ri Nuclear Test Site has been generally quiet. However, there is now one vehicle and a large contingent (70-100) of people standing in formation or watching in the courtyard of the Main Administrative Area," 38 North said, citing satellite imagery taken on Tuesday.

"Such a gathering hasn't been seen since January 4, 2013, which was followed by a nuclear test on Feb. 12," it said.

Other activity taking place at the site include the continued pumping of water out of the North Portal to keep it dry; the removal of material, probably rubble, and dumping on the tailings pile immediately to the east of the portal; and the probable removal of one or more vehicles or equipment trailers from in front of the portal, it said.

"The North Koreans know when commercial satellites are passing overhead and typically try to avoid activities during that time.

The fact these formations can be seen suggests that Pyongyang is sending a political message that the sixth nuclear test will be conducted soon," it said. "Alternatively, it may be engaged in a well-planned game of brinkmanship."

A new test would mark the North's sixth nuclear test. Pyongyang carried out its first nuclear test in 2006 and then conducted four more in 2009, 2013 and January and September of last year. (Yonhap)