The recent satellite imagery also showed that construction is continuing at the still incomplete light water reactor site at the Yongbyon complex, 38 North said.

"The construction adjacent to the reactor hall can now be identified as a transformer yard to connect the electricity producing reactor to the grid. The yard appears to be complete, but all the equipment is probably not yet installed," it said.

"Once finished, the North Koreans will have taken another step towards beginning initial operation of the reactor," it said.

The report also said that the North's five-megawatt nuclear reactor, which the communist nation has used to produce plutonium for nuclear bombs, "may not be operating or is only functioning at low power levels." It said there is "no evidence such as steam or hot water discharge" that would suggest the reactor is operating.

The reactor is believed to have provided Pyongyang with weapons-grade plutonium that the regime used in its three nuclear tests conducted in 2006, 2009 and 2013. The North has also built a uranium enrichment facility that gives Pyongyang a second way of building nuclear bombs in addition to its plutonium program.

The 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)