North Korea has restarted production of plutonium fuel, a senior U.S. State Department official said on Tuesday, showing that it plans to pursue its nuclear weapons program in defiance of international sanctions.
The U.S. assessment came a day after the U.N. nuclear watchdog said it had "indications" that Pyongyang has reactivated a plant to recover plutonium from spent reactor fuel at Yongbyon, its main nuclear complex.
The latest developments suggest North Korea's reclusive regime is working to ensure a steady supply of materials for its drive to build warheads, despite tightened international sanctions after its fourth nuclear test in January.
The U.S. official, who spoke on condition of anonymity, said that Washington is worried by the new plutonium reprocessing effort, but he offered no explicit word on any U.S. response.
"Everything in North Korea is a cause for concern," the official told Reuters.
"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," the official said. "That's what they're doing."
North Korea, which conducted its fourth nuclear test in January, vowed in 2013 to restart all nuclear facilities, including the main reactor and the smaller plant at Yongbyon, which was shut down in 2007 as part of an international disarmament-for-aid deal that later collapsed.
The International Atomic Energy Agency (IAEA), which has no access to North Korea and mainly monitors its activities by satellite, said last year it had seen signs of a resumption of activity at Yongbyon.
IAEA chief Yukiya Amano told a news conference in Vienna on Monday that there have been indications of renewed plutonium reprocessing activities at Yongbyon. Reprocessing involves extracting plutonium from spent reactor fuel, one route to obtaining bomb fuel other than uranium enrichment.
"I would agree that there are indications," the U.S. official said.
The official declined to confirm whether this determination was made from satellite imagery or intelligence sources, or to say how much plutonium North Korea could produce by this method.
South Korea's Unification Ministry spokesman Cheong Joon-hee said Seoul was closely watching movements related to the North's nuclear facility "with grave concern" but declined to comment directly on plutonium production.