"They've tampered with the field of the surveillance cameras so that we're unable to monitor the facilities," U.N. International Atomic Energy Agency (IAEA) spokesman Mark Gwozdecky told Reuters.

The IAEA has been carrying out limited monitoring activity in North Korea since the early 1990s, though it has never been allowed to conduct intrusive inspections aimed at fleshing out a secret weapons programme.

The IAEA has repeatedly called on Pyongyang to stick to the 1994 agreement with the United States and said that it "deplored" North Korea's insistence that it has a right to develop nuclear weapons.

"I have requested that the North Korean authorities allow immediately our inspectors to apply the necessary containment and surveillance measures...and not operate the reactor before the necessary safeguard measures are in place," IAEA chief Mohamed ElBaradei said in a statement released in Vienna.

ElBaradei also called on North Korea to take no further unilateral actions.

In 1994, the U.N. Security Council requested that the IAEA "take all the necesary steps it may deem necessary as a consequence (of the 1994 agreement) to monitor the freeze."

To monitor Pyongyang's compliance with the freeze, the IAEA sealed the five facilities covered by the freeze and installed permanent surveillance cameras. But on Saturday North Korea broke most of the seals and disabled the cameras.

In exchange for freezing its nuclear weapons programme, North Korea was to receive safer Western-built nuclear power plants that could not be used for a secret atomic weapons programme.

However, these facilities have not been built.