The UN's atomic watchdog says it has removed seals and surveillance cameras from part of North Korea's main nuclear complex at Pyongyang's request.
North Korea says the move is part of a plan to reactivate the Yongbyon plant, and that it plans to return nuclear material to the site next week.
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.
The removal of seals and cameras "was completed today" at the site, a spokeswoman for the International Atomic Energy Agency (IAEA) said.
IAEA inspectors will have no further access to the reprocessing plant, she added.
The US said North Korea's decision to exclude UN monitors was "very disappointing" and urged Pyongyang to reconsider the move or face further isolation.
"We strongly urge the North to reconsider these steps and come back immediately into compliance with its obligations as outlined in the six-party agreements," White House spokesman Gordon Johndroe said.
He said that Washington remained "open to further discussions" with the North on their obligations for denuclearisation.
The North has been locked in discussions for years over its nuclear ambitions with five other nations - South Korea, the US, China, Russia and Japan.
Symbolic gesture
Pyongyang began dismantling the reactor, which can be used to make weapons-grade plutonium, last November.
However, on Friday it announced that it was working to reactivate it.
North Korea 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.
Satellite view of nuclear site
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.
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.
