The finding by the Institute for Science and International Security in Washington follows similar findings by other agencies about the Communist country's main nuclear complex.
"Recent commercial satellite imagery shows that North Korea's Yongbyon nuclear center appears to be increasingly active," the ISIS said on its website. "There are several signs of new and continued activity and progress in the construction of facilities."
In October, South Korea's National Intelligence Service informed the country's lawmakers the 5 megawatt reactor, which was shut down in 2007, had been reactivated.
The ISIS report said a Digital Globe commercial satellite imagery taken earlier this week showed steam emerging from a building that years ago was involved in converting yellowcake to natural uranium dioxide.
"In other facilities in the complex, the uranium oxide was subsequently converted into metal and then made into fuel for the gas-graphite reactors, principally the 5 megawatt-electric reactor," the report said.
While the building's exact purpose cannot be determined simply from satellite imagery, one implication of the steam is that the building is operational as part of an effort to make fuel for the 5 megawatt reactor, the institute said.
Since earlier this year, North Korea has been expanding its gas centrifuge enrichment plant at Yongbyon, it said.
"This plant is believed to be producing low enriched uranium fuel for the light water reactor or for further enrichment to weapon-grade at Yongbyon or elsewhere," it said.
South Korea's Yonhap News Agency said the International Atomic Energy Agency last month had announced it also detected the release of steam and water from the Yongbyon reactor indicating it has been restarted.
The latest finding may further indicate North Korea is reverting to a provocation cycle after months of unanswered peace overtures toward the United States, Yonhap said.
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