South Korea's spy agency confirmed Tuesday that the North has restarted its Yongbyon reactor that had been mothballed since 2007.

The National Intelligence Service (NIS) told lawmakers from the National Assembly's Intelligence Committee that the five megawatt graphite moderated reactor has been in operation since around August.

The confirmation is the first to come out of the NIS, although there have been much speculation that the North had recently turned on the reactor again.

The reactor, located some 90 kilometers north of Pyongyang, has been cited for producing plutonium for the North's nuclear weapons program before it was shut down under a deal brokered at the six-party talks. The North had blown up the reactor's cooling tower in 2008 to show its seriousness about suspending operations.

Pyongyang, however, announced in early April when tensions spiked on the Korean Peninsula that it would restart work on the small reactor that does not generate any energy.

Independent sources that analyzed commercial satellite images have speculated since early last month that the reactor may be in operation. (Yonhap News)