North Korea is showing signs that it is preparing or has already begun reprocessing spent nuclear fuel to harvest plutonium for nuclear weapons, a U.S. website monitoring the communist nation said Tuesday, citing recent satellite imagery.

The assessment by the website 38 North is similar to that of the Institute for Science and International Security that last week cited as possible evidence of reprocessing activity the presence of container tanks in front of the reprocessing plant and smoke from a coal-fired plant providing steam for reprocessing activity.

38 North also cited the presence of two railroad flatcars at the reprocessing facility loaded "with casks or tanks that may be associated with chemicals used in a reprocessing campaign" and a small exhaust plume at the facility's thermal plant.

Both ISIS and 38 North reached their assessments based on commercial satellite imagery taken on May 22.

"Exactly how much new plutonium Pyongyang can produce if such a campaign is underway remains unclear given uncertainties about the level of operations at the 5 MWe reactor which restarted in 2013," 38 North said.

"At maximum capacity, the reactor could produce approximately 6 kilograms of plutonium per year, probably enough for two nuclear weapons," it said.

Earlier this year, Director of National Intelligence James Clapper said in a worldwide threat assessment that the North had restarted its 5-megawatt reactor and has since run it long enough to harvest plutonium "within a matter of weeks to months."

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods that, if reprocessed, could give the regime enough plutonium to make one bomb a year.

The reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its first three nuclear tests, in 2006,

2009 and 2013. The North conducted its fourth nuclear test on Jan. 6, claiming it successfully detonated a hydrogen bomb. (Yonhap)