North Korea has clearly restarted an aging plutonium reactor which would help it boost its nuclear weapons program, a US think tank said Wednesday after analyzing new imagery.

The US-Korea Institute at Johns Hopkins University, which last month reported signs that the Yongbyon reactor had resumed operation, said that more conclusive evidence emerged in the latest image.

A commercial satellite picture taken on September 19 showed the plutonium reactor releasing hot wastewater into a river through a new drainpipe, the institute said.

The dumping of the water "provides further evidence that Pyongyang has restarted its five-kilowatt reactor," analyst Nick Hansen wrote on the institute's blog, 38 North.

A picture from late July had not shown any sign of hot water discharge, indicating a recent relaunch, it said.

In reports released last month, the institute and another Washington think tank, the Institute for Science and International Security, had observed steam coming from the reactor.

The drainpipe is critical to maintaining a safe temperature at the reactor as North Korea knocked down a cooling tower in 2008 to show its commitment to a US-backed aid-for-disarmanent deal.

North Korea carried out its third nuclear test in February and two months later boasted that it would restart all facilities at Yongbyon to bolster its atomic arsenal.

Russia has warned that the resumption of Yongbyon could lead to catastrophe as the reactor was built in 1986 and is outdated.

The United States has also voiced concern, with the evidence reinforcing skepticism over North Korea's statements that the communist state is ready to return to negotiations.