Signs of activity at North Korea's mothballed Yongbyon nuclear reactor

North Korea's nuclear reactor at Yongbyon is releasing hot water, a sign operations have resumed at the facility capable of producing enough plutonium to make one atomic bomb a year.

Satellite imagery taken on September 19 shows water being released into the Kuryong River from the reactor facility at North Korea's main nuclear complex, according to the 38 North website, which is run by the US-Korea Institute at Johns Hopkins University's school of advanced international studies.

"This release of hot water indicates that the reactor is in operation and the turbine powered electrical generators are producing power," said Nick Hansen, who wrote the report.

If correct, the reactor will enable North Korea to expand its nuclear weapons capabilities in defiance of world powers and the United Nations. Last month, the International Atomic Energy Agency called on North Korea to halt its nuclear activities, including weapons tests and the restarting of the reactor.

Previously, Hansen had reported that satellite images taken on August 31 seemed to indicate reactor activity because they showed white steam rising from a building containing turbines and generators powered by the reactor. That was the first sign that a start-up process appeared to be under way, since there is no independent confirmation from Kim Jong-un's regime.

"We can't be certain" that the reactor has been restarted, "but would there be smoke from an unlit chimney?" said South Korean Defence Ministry spokesman Kim Min-seok.