A U.S.-based research group says North Korea has likely made "a more wide-ranging, extensive" effort to restore its main nuclear site than previously thought.
The analysis by the U.S.-Korea Institute at Johns Hopkins University is based on recent satellite images of the Yongbyon nuclear complex in the country's northwest.
A five-megawatt plutonium reactor at the compound was restarted in August, weeks after Pyongyang vowed to expand its nuclear weapons stockpile.
Although recent satellite images showed activity had resumed, it was unclear how quickly the reactor could resume making plutonium for nuclear bombs, since a lengthy process to make fresh fuel rods first needed to be completed.
But a piece published Tuesday on the U.S.-Korea Institute's 38 North blog suggested Pyongyang had anticipated this and may have been working on fuel production facilities for years.
The institute pointed to evidence including white stains on a roof and a nearby dumping grounds it said are consistent with the fuel rod production process.
When operating normally, analysts say the reactor will be able to produce enough plutonium to make about one or two bombs per year. Pyongyang is believed to already have enough to make up to 10 bombs.
The U.S. Korea Institute's satellite imagery analysis also identified another building in the Yongbyon complex that may be related to the production of fuel assemblies for a light-water uranium reactor.
Uranium would give North Korea a second, and potentially easier method of producing nuclear weapons, though little is known about how much this program has advanced.
North Korea conducted nuclear tests in 2006, 2009 and in February of this year. All were in violation of UN sanctions and received near universal condemnation.
Last week, South Korea said there is evidence the North is making steady preparations for a fourth nuclear test, but said that no such move appears imminent.