North Korea said in April it had begun reprocessing the fuel rods
North Korea has completed the reprocessing of spent nuclear fuel rods to extract weapons-grade plutonium, according to its official news agency.
The Korean Central News Agency (KCNA) added that "noticeable successes" had been made in turning the extracted material into weapons-grade plutonium.
The statement comes one day after North Korea said it was ready for talks with the US on its nuclear programme.
It also threatened to "go its own way" if Washington refused direct talks.
Eight thousand spent fuel rods had been reprocessed, KCNA said. Analysts say this is enough plutonium to make at least one atomic bomb.
North Korea was already believed by some analysts to have enough weapons-grade plutonium for about six bombs, but had none small enough to place on a missile.
"Noticeable successes have been made in turning the extracted plutonium weapon-grade for the purpose of bolstering up the nuclear deterrent in the DPRK [North Korea]," said the KCNA statement.
The announcement is the latest move in stop-start attempts by the US and its allies to end North Korea's nuclear programme.
North Korea said on Monday that it was ready to return to six-party talks on rolling back its nuclear programme if the US first agreed to hold bilateral talks.
The six parties are the two Koreas, the US, China, Japan and Russia.
The US has repeatedly said bilateral talks can be held as part of the six-party process.
The forum had reached agreement in 2007 that North Korea would shut down the Yongbyon nuclear complex in return for fuel aid and political concessions for the isolated country.
But the deal unravelled late last year amid a disagreement over how to verify North Korea's account of its atomic activity.
When North Korea's launch of a long-range rocket in April brought UN censure, Pyongyang announced it was restarting the Yongbyon reactor and would begin to reprocess the spent fuel rods.
An underground nuclear test, the second since 2006, triggered UN sanctions.
In September, the North said it had entered the final phase of enriching uranium, which would give it a second path to making nuclear weapons.
In its latest statement, KCNA said the UN censure of what it called a peaceful attempt to launch a satellite was "a grave insult to the dignity of its people".
North Korea "was compelled to take measures for bolstering up its deterrent for self-defence to cope with the increasing nuclear threat and military provocations of the hostile forces," the statement said.