The United States views North Korea's actions with "great concern," the State Department said Wednesday, following reports that the communist nation has begun reprocessing spent nuclear fuel to harvest plutonium.

A senior State Department official was quoted by Reuters as saying Tuesday that the North took spent fuel from the 5-megawatt reactor at its Yongbyon nuclear complex and took it to the reprocessing facility to produce the element that can be used for nuclear weapons.

"We have been very clear that we view North Korea's rhetoric and actions with great concern. We continue to pursue action with our regional and international partners, as well as international bodies," Ory Abramowicz, a State Department spokesman, told Yonhap News Agency.

The spokesman, however, declined to comment on the report.

"We refer you to the concerns Director of National Intelligence James Clapper expressed in February during Congressional testimony about North Korea's potential to reprocess plutonium," Abramowicz said. Clapper said at the time that the North had run the 5-megawatt reactor long enough to harvest plutonium "within a matter of weeks to months."

"We call on North Korea to refrain from actions and rhetoric that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international commitments," Abramowicz said.

Later in the day, State Department Deputy Spokesman Mark Toner also expressed concern.

"I don't want to get into too much detail because it veers very quickly into intelligence. But of course, speaking broadly about the issue, we're concerned that North Korea could have access to materials that would allow it to produce nuclear weapons, more nuclear weapons," he said at a regular press briefing.

Earlier this week, 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 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.

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)