North Korea appears to have restarted plutonium production for its nuclear weapons program at its main Nyongbyon reactor complex, commercial satellite imagery released by a think tank showed Friday.

Imagery from a previous analysis by the North Korean-watching website 38 North on Jan. 18 showed that Pyongyang had been preparing to restart the reactor after spent fuel rods had been unloaded for reprocessing to produce extra plutonium for its weapons program, the report said.

"Imagery from January 22 shows a water plume (most probably warm) originating from the cooling water outlet of the reactor, an indication that the reactor is very likely operating," the report said, noting that most of the river remained frozen except where the warm water mixed.