North Korea appears to be trying to restart its main nuclear reactor after idling the facility for about five months, a U.S. think tank said Wednesday, citing steam and other indications.

Commercial satellite imagery taken from Dec. 24 through Jan. 11 indicates new activity at the five-megawatt reactor at the North's Yongbyon nuclear complex, said 38 North, a website run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies.

"One possibility is that the North is in the early stages of an effort to restart the reactor. However, since the facility has been recently observed over a period of only a few weeks, it remains too soon to reach a definitive conclusion on this," it said.

The reactor had been shut down from late August until mid-December, it said.

Indications of possible efforts to restart the facility include snow melting off the reactor's roof and steam coming from a probable pressure relief valve just before it enters the turbine building next to the reactor, the website said.

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods which, if reprocessed, could give the regime enough plutonium to make one bomb a year.

In addition to the 5-megawatt reactor, the North has also been building a larger-scale light water reactor at Yongbyon that experts say could give Pyongyang enough plutonium to make about five or six weapons a year.

The North has conducted underground nuclear tests three times in 2006, 2009 and 2013, and also built a facility to enrich uranium, which gives the communist regime a second way of building nuclear bombs in addition to its plutonium program.

Six-party talks aimed at ending Pyongyang's nuclear program have been stalled since the last session in late 2008. North Korea has called for resuming negotiations without preconditions, but the U.S. has demanded Pyongyang first take concrete steps demonstrating its denuclearization commitments. (Yonhap)