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North Korea continues to operate its key uranium plant, a U.S. think tank said Monday, suggesting a steady growth in North Korea's stockpile of the material used to build nuclear weapons.

Citing recent satellite imagery, Beyond Parallel, a project of the Washington-based Center for Strategic and International Studies, reported the North's Pyongsan Uranium Concentrate Plant remains operational and is producing uranium concentrate.

"The main plant of the Pyongsan Uranium Concentrate Plant continues to show activity consistent with the continued production of uranium concentrate and ongoing maintenance efforts," the report said, citing commercial satellite imagery collected between April and October.

It said some panels on the roof of a "leaching and classifying building" at the Pyongsan plant appeared to have been replaced due to deterioration that it said "is caused by acid vapor condensing on the underside of the roof panels."