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People watch a TV showing an image of North Korea's new guided missile during a news program at the Suseo Railway Station in Seoul, March 26. Recent satellite imagery showed steam from the plutonium reprocessing plant at North Korea's Yongbyon nuclear complex, a U.S. think tank said on March 31. AP

Recent satellite imagery showed steam from the plutonium reprocessing plant at North Korea's Yongbyon nuclear complex, a U.S. think tank said Wednesday, an indication of activity at the facility used to extract the key nuclear weapon ingredient.

The imagery taken on Tuesday shows a plume of steam or smoke emanating from a small support building in the center of the reprocessing plant, known as the Radiochemical Laboratory, according to Beyond Parallel, a project of the Center for Strategic and International Studies think tank.

The facility is used to reprocess spent fuel rods to extract plutonium, a fissile material used to build nuclear weapons along with highly enriched uranium. About 6 kilograms of plutonium is required a build a nuclear bomb.

In recent weeks, steam has also been observed at a thermal plant used to provide heat to the reprocessing facility, spurring speculation that the North might be preparing to harvest plutonium. The latest imagery also detected steam from the plant, the report said.

"This, while not an indicator of a reprocessing campaign itself, indicates that the building is occupied and being heated," the think tank said.