According to the US State Department, North Korea has restarted production of plutonium fuel, indicating that it intends to pursue its nuclear weapons program in defiance of international standards, and tougher UN sanctions that were backed by China in March.
The latest developments suggest North Korea's regime is working to ensure a steady supply of materials for its drive to build warheads, Reuters reports.
The US assessment comes a day after a UN 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 website 38 North reported last week, based on commercial satellite imagery, that exhaust plumes had been detected twice in May from the thermal plat at Yongbyon's Radiochemical Laboratory, the site's main reprocessing installation. The Institute for Science and International Security also confirmed reports of exhaust emissions from a chimney at the plant.
"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, that's what they're doing." the US offical said, speaking on condition of anonymity.
The country announced last month at a congress of its ruling Workers' Party that it would strengthen its defensive nuclear weapons capability, and although in the past it had obtained key components for its nuclear program from other countries, there was no sign of any recent outside procurement involved in reactivating its plutonium reprocessing said the US official.
There is little proven knowledge about the quantities of weapons-grade uranium or plutonium that North Korea possesses, or its ability to produce either.
There is little proven knowledge about the quantities of weapons-grade uranium or plutonium that North Korea possesses, or its ability to produce either, though plutonium from spent fuel at Yongbyon is widely believed to have been used in its nuclear bombs.
South Korean Defense Minister Han Min-koo said last month the North probably had about 40 kg (88 lb) of plutonium. That would be enough to make eight to 10 bombs, according to experts.
Operating the 5 megawatt reactor could yield about 5-6 kg of plutonium a year, they said.
Experts at the U.S.-Korea Institute at Johns Hopkins University's School of Advanced International Studies in Washington predicted last year that North Korea's nuclear weapons stockpile could grow to 20, 50 or 100 bombs within five years, from an estimated 10 to 16 weapons at that time.
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", and a spokesman for China's Foreign Ministry said "we hope all parties can work hard together to put the nuclear issue back on the track of dialogue and negotiations."
North Korea has already declared itself "a responsible nuclear weapons state" and disavowed the use of nuclear weapons unless its sovereignty is first infringed by others with nuclear arms.
CNN reported in 2015 that North Korea was one of the few countries to have nuclear weapons.