North Korea's Yongbyon nuclear complex operating at low power

SEOUL, Jan. 14 (UPI) -- North Korea's nuclear reactor, capable of producing plutonium, is running at low power, a U.S. think tank stated Wednesday.

The Institute for Science and International Security noted the latest imagery from Jan. 11 showed steam rising from a 5-megawatt reactor at Yongbyon nuclear complex, South Korean news agency Yonhap reported.

Think tank experts also saw signs a gas centrifuge plant for enriching uranium was working – snow on the roofs of the plant's main buildings had melted, The Japan Times reported.

But a collective analysis of the satellite imagery, dating from late 2014 to late 2015, showed that the reactor at best has run sporadically.

"It is, therefore, very likely that the reactor is operating intermittently or at low power as of January 2016," the analysts wrote.

The organization's findings challenge previous North Korea claims that normal operations at Yongbyon had resumed last September.

Yongbyon is North Korea's graphite-moderated reactor and the source of weapons-grade plutonium. When operating at full capacity, Yongbyon can produce about 13 pounds of plutonium annually, just enough to produce one nuclear bomb, according to experts.

North Korea also has completed a uranium enrichment facility.

The think tank said the enrichment plant could be up and running, given the snow has melted away on the roof of the plant. The heat from inside of the building could mean centrifuges are in operation.

The United States has grown increasingly concerned about North Korea's nuclear weapons program since Pyongyang announced a "successful" hydrogen bomb test in early January.

President Barack Obama and Russian President Vladimir Putin agreed Wednesday by phone that tougher measures against the North are needed.

USA Today reported the two leaders condemned the test, calling it a violation of U.N. Security Council resolutions.