The United States 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 International Atomic Energy Agency 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.
A similar step in 2002 sparked a crisis which eventually resulted in Pyongyang testing a nuclear weapon in 2006.
UN nuclear inspectors have been expelled from the most sensitive part of the Yongbyon facility; a 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 IAEA said that Pyongyang intended to introduce nuclear material at the facility next week.
IAEA inspectors had been observing the process to close Yongbyon - as part of a deal reached in six-party talks with China, Japan, Russia, South Korea and the US.
Dr 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."
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 a disarmament deal 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.