The United States on Saturday blasted North Korea's decision to reactivate its nuclear program and called for the reclusive nation to return to international denuclearization talks.
"We will not accept North Korea as a nuclear-weapons state," said Megan Mattson, a State Department spokeswoman.
The North's announcement that it had started reprocessing spent fuel rods to make weapons-grade plutonium came just hours after the United Nations slapped sanctions on three North Korean firms accused of backing missile development.
On April 14, the North said it would quit six-nation nuclear disarmament talks and restart its atomic weapons program after the UN Security Council issued a statement condemning the Pyongyang's controversial April 5 rocket launch.
Pyongyang says it put a satellite into orbit but the United States and its allies say the launch was a disguised long-range ballistic missile test.
"The United States remains committed to the six-party goal of the complete and verifiable denuclearization of the Korean Peninsula in a peaceful manner through the six-party talks," said Mattson.
Mattson said the US continues "to seek full implementation of the September 19, 2005 Joint Statement under which North Korea committed to abandon all nuclear weapons and existing nuclear programs and return, at an early date, to the Treaty on Non-Proliferation of Nuclear Weapons and to IAEA safeguards."
Infuriated by the UN's actions, North Korea has expelled inspectors from the United States and the International Atomic Energy Agency (IAEA) who had been monitoring its stated efforts to dismantle its nuclear programs.
The North had been disabling parts of the Yongbyon nuclear complex as agreed under a February 2007 six-nation deal involving the two Koreas, the United States, China, Russia and Japan.
But six-party negotiations stalled last December because of disputes about ways to verify its declared nuclear activities.
Analysts say it will take three to four months before the North completes reprocessing some 8,000 spent fuel rods from the reactor in Yongbyon to obtain plutonium.
The North, which carried out its first nuclear test in October 2006, reportedly put the size of its plutonium stockpile at 31 kilograms when it handed over a nuclear declaration in June 2008.
If all has been turned into weapons, the North might have six to eight bombs, experts say.