A U.S. research institute says recent satellite images shows construction work at North Korea's nuclear complex, but also signs that its existing reactor is not fully operating.

The Nyongbyon (YAWNG-bee-yon) facility is North Korea's only publicly known nuclear complex. International monitors were expelled in 2009, but commercial satellite photos offer a partial window into its operations.

The U.S.-Korea Institute says an image from late May shows construction next to the uranium enrichment facility, and work on what appears to be a structure related to the electricity supply of a still-incomplete light-water reactor.

The existing plutonium reactor appears to be operating at low power levels or was not operating on the day the image was taken. The reactor can be used to produce fissile material for nuclear weapons.