The United States says North Korea's plutonium production is contrary to its nuclear disarmament commitments and violates resolutions passed by the U.N. Security Council.
North Korea announced Tuesday it has produced more material for use in nuclear weapons and has made substantial progress in turning plutonium into fuel for nuclear bombs.
A report in the North's official Korean Central News Agency says the country completed reprocessing 8,000 spent fuel rods at its Yongbyon nuclear facility.
U.S. State Department spokesman Ian Kelly says North Korea's actions violate U.N. resolutions.
"I think, you know, what we're focused on with - on North Korea is getting to the point where we can relaunch the six-party talks, which will get us to our ultimate goal, which is the denuclearization of the Korean Peninsula," said Ian Kelly. "I will say as a matter of principle that reprocessing plutonium is contrary to North Korea's own commitments that it committed to in the 2005 joint statement, and also would be a violation of various U.N. Security Council resolutions."
In 2005 North Korea agreed to scrap its nuclear program in return for security and diplomatic guarantees and energy aid.
The Obama administration has been trying to convince Pyongyang to return to multilateral negotiations involving the United States, North Korea, South Korea, China, Japan and Russia.
North Korea quit the six-party talks earlier this year after the United Nations criticized its launch of a long-range missile.
In May, North Korea carried out its second atomic weapons test.
Spokesman Ian Kelly declined to condemn the latest moves, but said nothing should be done to raise tensions in Northeast Asia.
"We need to keep our eyes on the goal here, and that's a comprehensive peaceful solution to the tension in Northeast Asia," he said. "And we want to make sure that we all move toward that goal. And it's just that I think everybody should be careful and ratchet down the rhetoric and not take any actions that would contribute to tension in the region."
North Korea's announcement came a day after the country's foreign ministry pressed the United States to agree to direct talks. The ministry said if Washington is not willing to negotiate, Pyongyang will go its own way, an apparent threat to boost its nuclear arsenal.
Kelly reiterated the U.S. is willing to engage in such discussions.
"We remain willing to talk to them bi-laterally within the context of the six-party talks, which means with the support, and in consultation, with our partners in these talks," said Kelly. "It's just that we still have not decided on when and where we will have these bilateral talks."
Pyongyang has said it needs nuclear weapons to defend itself against the United States.
Washington says it has no intention of attacking North Korea.