North Korea, which agreed to deconstruct the plutonium-producing facility as part of a 2007 six-nation deal in return for economic incentives, has accused the United States of reneging on its pledges under the agreement.
International Atomic Energy Agency spokeswoman Melissa Fleming told reporters in Vienna: "From here on the IAEA inspectors will have no further access to the reprocessing plant... They [North Korea] also informed IAEA inspectors that they plan to introduce nuclear material to the reprocessing plant in one week's time."
IAEA deputy head Olli Heinonen said the inspectors had been forced to remove seals and surveillance equipment from the facility.
Pyongyang announced in late August an immediate halt to work on dismantling the Yongbyon reactor, after accusing the United States of failing on its promise to remove North Korea from its list of state sponsors of terrorism, which keeps the country in financial isolation.
North Korea insists it has fully met its denuclearization commitments under the six-party talks, which involve the two Koreas, China, Japan, Russia and the United States, and has demanded that the United States end its "hostile policies."
The reclusive communist regime began disabling the reactor and other facilities at Yongbyon last November. However, Washington insists that the North accept strict procedures to verify the declaration it made in June of its nuclear activities before it can be taken off the blacklist.
