The finding is based on analysis of recent commercial satellite imagery at the North's main Nyongbyon nuclear facility.
The plant is used to reprocess spent fuel from a 5 megawatt reactor that has produced plutonium used for past nuclear test explosions. Last year, to international alarm, North Korea restarted the reactor that had been shuttered under a 2007 disarmament agreement.
The US-Korea Institute at Johns Hopkins School of Advanced International Studies says the reactor appears to have been shut down for about 10 weeks, longer than normal maintenance would require.
Steam coming from buildings associated with the reprocessing plant suggests it is preparing to commence operations, according to the analysis by Nick Hansen, a satellite imagery specialist who closely monitors developments in the North's weapons programs.
Also Read
There are also signs of chemical waste outside a fuel fabrication facility, suggesting new fuel rods are being produced to replace the spent ones, he writes.
He said the amount of plutonium that would be harvested from a few spent rods would be small but this could be a harbinger of a bigger reprocessing effort when more fuel rods inside the reactor are spent.
"This may be a sign of things to come, perhaps a year from now, when the North Koreans unload all of the fuel from their reactor and produce a few additional bombs worth of plutonium," he said.
The reactor can produce about one bomb's worth of plutonium per year, it said.
North Korea also has a uranium enrichment facility at Nyongbyon, likely giving it a second method to produce fissile material for bombs.
The North has conducted three underground nuclear tests, the latest in February 2013.
A North Korean envoy yesterday threatened further nuclear tests in response to moves at the UN for the country's harsh human rights situation to be referred to the International Criminal Court.