South Korea's spy agency has confirmed that the North has rekindled its Yongbyon reactor deactivated since 2007.
The National Intelligence Service reported that the five Megawatt graphite moderated reactor has restarted operations.
Reports about activities at Yongbyon made the headlines last week, with satellite images showing hot water being released into the Kuryong River from the reactor facility.
The 38 North website, which is run by the US-Korea Institute at John Hopkins University's school of advanced international studies, had said: "This release of hot water indicates that the reactor is in operation and the turbine powered electrical generators are producing power."
The reactor will enable Pyongyang 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.
The development came after North Korean leader Kim Jong-un ordered Pyongyang's army to be on high alert as a US-led, three-day naval exercise was set to begin in the Sea of Japan.
Both South Korea and Japan are set to participate in the joint naval drill which have been slightly delayed due to bad weather. Seoul said the manouvres were regular maritime exercises.
North Korea recently threatened to carry out "ruthless" pre-emptive strikes against the US and South Korea.