U.S. experts say North Korea has resumed operations at its main plutonium producing reactor in North Pyongan Province.

Researchers Jack Liu and Joseph Bermudez said they reached the conclusion based on analysis of commercial satellite imagery of the Yongbyon nuclear complex last Sunday.

In a report posted Friday on the 38 North Web site, the researchers said a water plume was coming from the cooling water outlet of the reactor, calling it an indication that the reactor is “very likely operating.” They pointed out most of the river was frozen except where the water from the reactor mixes with the river.

They said although they can’t estimate at what power level the reactor is running, as it is difficult to measure water temperature or flow from the reactor, the operation is “considerable.”

South Korea’s Joint Chiefs of Staff(JCS) has said the North has shown no unusual moves yet. However, the JCS said it is keeping a close eye on the North’s military as it sees the possibility of a provocation, such as a nuclear test or a missile launch, whenever its leadership orders it.

Since the start of the year, the North has threatened to test-fire an intercontinental ballistic missile(ICBM). 

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