The United States said Wednesday North Korea's resumption of nuclear fuel reprocessing for plutonium production is a "clear violation" of UN Security Council resolutions.

Pyongyang's Atomic Energy Institute said in an interview with Japan's Kyodo News published earlier Wednesday that the communist nation has "reprocessed spent nuclear fuel rods removed from a graphite-moderated reactor." It was referring to the five-megawatt reactor at the North's Yongbyon nuclear complex.

It marked the first time the North has confirmed resumption of nuclear reprocessing, though private U.S. research institutes have reported signs in satellite imagery suggesting that reprocessing has begun. The UN nuclear watchdog, IAEA, also reported such indications.

"If these reports are correct, it is a clear violation of UNSC resolutions which prohibit such activities," State Department spokeswoman Katina Adams told Yonhap News Agency. "These actions only serve to increase the international community's resolve to counter the DPRK's prohibited activities, including through implementing existing UN Security Council sanctions."

According to Kyodo, the North's atomic agency did not rule out the possibility of conducting a fifth nuclear test and claimed that the country has already succeeded in "minimizing, making lighter and diversifying" nuclear weapons.

"Under conditions that the United States constantly threatens us with nuclear weapons, we will not discontinue nuclear tests," the agency was quoted as saying.

It also said the country will build a 100,000-kilowatt light-water nuclear reactor for experimental use.

Yongbyon's 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.

In February, 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 US reiterated its security commitment to South Korea.

"Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad. We remain prepared to defend ourselves and our allies," Adams said.

She also urged the North to refrain from actions and rhetoric that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international obligations and commitments.

"We continue to pursue action with our regional and international partners, as well as international bodies," she said. (Yonhap)