SEOUL, South Korea — North Korea may be attempting to restart its main nuclear bomb fuel reactor after a five-month shutdown, a U.S. research institute said Thursday.

If true, the finding, which is based on recent commercial satellite imagery, will be an added worry for the United States and the North’s neighbors at a time of increasing animosity over recent U.S. sanctions against the North and Pyongyang’s fury about a U.N. push to punish its alleged human rights abuses.

Activity at the 5-megawatt Nyongbyon reactor is closely watched because North Korea is thought to have a handful of crude nuclear bombs, part of its efforts to build an arsenal of nuclear-tipped missiles that could one day hit America’s mainland. Nyongbyon, which has produced plutonium used for past nuclear test explosions, restarted in 2013 after being shuttered under a 2007 disarmament agreement. It has been offline since August.