North Korea has begun reprocessing spent nuclear fuel to produce plutonium that can be used for nuclear weapons, a senior U.S. State Department official was quoted as saying Tuesday.

"They take the spent fuel from the 5 megawatt reactor at Yongbyon and let it cool and then take it to the reprocessing facility, and that's where they've obtained the plutonium for their previous nuclear tests. So they are repeating that process," Reuters quoted the unidentified official as saying.

"That's what they're doing," the official said on condition of anonymity, according to Reuters.

Comment from the State Department was not immediately available.

The report came a day after International Atomic Energy Agency Director General Yukiya Amano said there are indications that the North could have started nuclear reprocessing.

Private U.S. research institutes, citing commercial satellite imagery, have also 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 reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its first three nuclear tests, in 2006, 2009 and 2013. The North conducted its fourth nuclear test on Jan. 6, claiming it successfully detonated a hydrogen bomb. (Yonhap)