The UN's nuclear watchdog says North Korea has banned its inspectors from entering the Yongbyon nuclear complex.
The International Atomic Energy Agency (IAEA) said it could no longer monitor the reprocessing plant.
The US State Department said the move was not positive or helpful, and urged Pyongyang to reverse its decision.
Last month, Pyongyang announced it would reactivate Yongbyon because it said the US had not fulfilled its part of an international disarmament deal.
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The agency said its staff were permitted to remain on the site but only in their quarters and were "no longer allowed to carry out the monitoring and verification of any nuclear activity whatsoever".
The IAEA also said it had been told by Pyongyang that disablement work on the facility had been stopped.
'Not helpful'
US State Department spokesman Sean McCormack said that North Korea's actions were "not positive, not helpful".
But he said that the steps were reversible and urged Pyongyang to "take a different set of decisions".
US Secretary of State Condoleezza Rice said she was reviewing the situation and discussing it with colleagues.
"Let's just wait and see over the next several days," she told reporters in Washington.
IAEA inspectors had been observing the process to shut down Yongbyon under a deal between North Korea and five other states for Pyongyang to abandon its nuclear activities in exchange for diplomatic incentives and aid.
Pyongyang began the process of dismantling the reactor last November and blew up the main cooling tower in a symbolic gesture of its commitment to the process.
But last month, Pyongyang said it would take steps to restore the Yongbyon reactor to plutonium production after the US did not remove North Korea from a list of countries it says supports terrorism.
Washington has said it will not do so until procedures by which the disarmament could be verified were established.
Experts say Yongbyon could take up to a year to bring back into commission, so there will be no new plutonium production for a while.
But the IAEA's announcement news comes a day after a senior South Korean military official said the North was trying to develop a nuclear warhead that would fit on to a missile.
Gen Kim Tae-young said Pyongyang was believed to have enough plutonium to make six or seven warheads but it was not clear if they had succeeded in developing a warhead.
The US has warned North Korea to "avoid any steps that increase tension on the peninsula".
(BBC)
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