The US has urged North Korea to reconsider its decision to bar UN monitors from its main nuclear complex.
Secretary of State Condoleezza Rice said restarting its nuclear programme would deepen Pyongyang's isolation, but she added that talks were not dead.
The UN's atomic watchdog earlier said it had removed seals and surveillance cameras from part of the Yongbyon plant at North Korea's request.
The move comes amid a dispute over an international disarmament-for-aid deal.
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The UN nuclear inspectors have been expelled from the most sensitive part of the Yongbyon facility; the reprocessing plant which can be used to make weapons-grade plutonium.
North Korea requested the action on Friday, saying it intended to reactivate the plant.
The International Atomic Energy Agency (IAEA) said that Pyongyang intended to introduce nuclear material at the facility next week.
Its inspectors had been observing the process to shut down Yongbyon - as part of a deal struck in six-party talks with South Korea, the US, China, Russia and Japan.
Ms Rice said that discussions with the North on its obligations for denuclearisation were "by no means" dead.
"We've been through ups and downs in this process before. The important thing is that this is a six-party process and that means there other states that are carrying the same message," she said.
Symbolic gesture
Pyongyang began dismantling the reactor last November.
It was expecting to be removed from the US terror list after submitting a long-delayed account of its nuclear facilities to the international talks in June, in accordance with the disarmament deal it signed in 2007.
It also blew up the main cooling tower at Yongbyon in a symbolic gesture of its commitment to the process.
However, the US said it would not remove North Korea from its list of state sponsors of terrorism until procedures by which the North's disarmament would be verified were established.
The North has been locked in international discussions for years over its nuclear ambitions.
North and South Korea have been technically at war since their 1950-53 conflict ended without a peace treaty.
Fuel rods
Experts say the Yongbyon plant could take up to a year to bring back into commission, so there will be no new plutonium production for a while.
However, there is plenty already available in the form of the spent fuel rods, taken from the reactor core, but only removed to a water-cooled tank on the site, says the BBC's John Sudworth in Seoul.
It is this nuclear material that will now be introduced into the separate plutonium reprocessing plant, according to the information given to the IAEA.
Some estimates suggest the fuel rods could yield about 6kg (13lbs) of plutonium within two to three months - enough for one atomic bomb to add to North Korea's existing stockpile.
(BBC)
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