North Korea could have started reprocessing spent nuclear fuel to harvest plutonium for nuclear weapons, the International Atomic Energy Agency said Monday.

"The DPRK (North Korea) resumed the activities of five megawatt reactors or enrichment or reprocessing," IAEA Director General Yukiya Amano said at a press briefing in Vienna, referring to facilities at the North's Yongbyon nuclear complex.

"However, as we do not have inspectors on the ground, we are only observing through satellite imagery. We cannot state for sure, but we have indications of certain activities through the satellite imagery," he said.

Citing commercial satellite imagery, private U.S. research institutes have recently reported signs of the North running or preparing to run the reprocessing plant, such as smoke from a coal-fired plant providing steam for reprocessing activity.

Earlier this year, Director of National Intelligence James Clapper said in a worldwide threat assessment that the North had restarted its 5-megawatt reactor and has since run it long enough to harvest plutonium "within a matter of weeks to months."

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods that, if reprocessed, could give the regime enough plutonium to make at least one bomb a year.

The IAEA chief expressed concern about the North's activities.

"I remain seriously concerned about the nuclear program of the Democratic People's Republic of Korea. It is deeply regrettable that the DPRK has shown no indication that it is willing to comply with the Security Council resolution adopted in response to its nuclear test earlier this year," he said during a Board of Governors meeting.

"The agency remains ready to contribute to the peaceful resolution of the DPRK nuclear issue by resuming its verification activities once a political agreement is reached among countries concerned. I call upon the DPRK to comply fully with its obligations under relevant Security Council resolutions, to cooperate promptly with the agency in implementing its NPT Safeguards Agreement, and to resolve all outstanding issues," he said. (Yonhap)