A North Korean nuclear reactor has recently shown signs of operation, suggesting the regime has resumed production of plutonium for its nuclear weapons program, a U.S. website said Monday.

38 North, which monitors the reclusive state, said commercial satellite imagery from Feb. 25 showed activity around the 5 MWe reactor at the Yongbyon Nuclear Scientific Research Center.

"A steam vapor plume was observed at the 5 MWe reactor on imagery from both Feb. 17 and 25," the website said. "Such plumes have generally been a useful indicator of reactor operations, however, they were usually accompanied by a cooling water discharge near the river outfall."

The absence of cooling water discharges in images from both dates could mean the vapor plume is unrelated to reactor operations. But more likely, the website said, it could mean the outfall pipeline has been extended further into the river.

"The presence of ice melt on the river supports the conclusion that the reactor is indeed operating and that the outfall pipeline has been extended," according to 38 North. "If the reactor is operating again, as the evidence suggests, it means North Korea has resumed production of plutonium presumably for its nuclear weapons program."

If the pipeline has been extended, monitoring efforts could become more difficult going forward, the website said, adding that an alternative means would be low resolution thermal infrared satellite imagery. (Yonhap)