North Korea's plutonium-producing nuclear reactor appears to be still running intermittently or at low power while its uranium-enrichment plant is also in operation, a U.S. research institute said Wednesday, citing recent satellite imagery.

The Institute for Science and International Security said the Jan. 11 imagery shows steam coming from the 5-megawatt reactor building at the Yongbyon nuclear complex, but no sign of a steady stream of water being discharged from it. 

Steam indicates the reactor is in operation while water discharge would be a sign of full-scale operation, it said.

"It is, therefore, very likely that the reactor is operating intermittently or at low power as of January 2016," it said.

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 last week, claiming it successfully detonated a hydrogen bomb.

The reactor is believed to have been under retrofitting or upgrading in recent years.

In addition to its plutonium program, the North has also built a uranium enrichment facility.

ISIS said the enrichment plant is believed to be in operation as the imagery shows no snow on the roof of the plant. That is believed to be due to heating inside the building aimed at ensuring that the centrifuge functions correctly, it said.

The North has also been building a light water nuclear reactor, but ISIS said that the satellite imagery showed no indications that it is in operation.(Yonhap)