A U.S. research institute says North Korea may be attempting to restart its main nuclear bomb fuel reactor after a five-month shutdown.

If true, the finding Thursday, will be an added worry for the United States and the North's neighbors at a time of increasing animosity.

The Nyongbyon reactor restarted in 2013 after being shuttered under a 2007 disarmament agreement. It has been offline since August.

Possible signs in satellite imagery from Dec. 24 through Jan. 11 that the reactor is being restarted include hot water drainage from a pipe at a turbine building and growing snow-melt on the roofs of the reactor and turbine buildings.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says more monitoring is needed to reach a definitive conclusion about what's happening.