North Korea is forging ahead with construction of a new nuclear facility believed to be designed to separate isotopes from spent fuel, such as tritium, a key ingredient for hydrogen bombs, a U.S. research institute said.

The Institute for Science and International Security said in a report released Monday that satellite imagery taken on Jan. 25 of the North's Yongbyon nuclear complex shows "external construction signatures" at the new facility site, such as vehicles, construction material, and earth displacement activities.

"The precise purpose of this site remains unknown. However, it is of interest because the signatures visible through a historical analysis of satellite imagery are consistent with an isotope separation facility, including tritium separation," ISIS said.

"This assessment is shared not only by an expert ISIS consulted but also by a government expert we consulted who has long experience in assessing activities at the Yongbyon site," it said.

In September, ISIS first raised the possibility that the new facility could be an isotope separation plant. 

If confirmed as such, the facility would separate isotopes from spent nuclear fuel rods from the 5-megawatt reactor, the North's main plutonium-producing reactor, the institute said at the time.

Though the reactor is not an ideal producer of isotopes, it can be used in that way, it added.

It also said the North's focus could be to produce tritium, one of ingredients for hydrogen bombs.

North Korea carried out its fourth nuclear test on Jan. 6, claiming that it tested a hydrogen bomb. Experts have cast doubts over the claim, pointing out that the yield is too low.

ISIS also said that the latest satellite imagery showed the North's 5-megawatt reactor is operational, albeit periodically and at reduced power, as snow near the reactor's discharge pipeline has melted, a sign that the reactor discharged warm water through the pipeline.

The image showed signs that the North's new light water reactor has not yet been operational.

"The delay in the reactor's operation may be a consequence of the fact that it is no longer a priority," ISIS said. "However, it is possible that North Korea may have also decided to redesign the reactor."

The institute also said that the uranium enrichment plant at the complex appears to be also operational, citing the absence of snow on several roofs, including the roof of the main centrifuge building. (Yonhap)