WASHINGTON - The United States accused North Korea on Tuesday of violating a six-nation nuclear accord and maintained it on a terror blacklist, after the hardline communist state defiantly stopped disabling its atomic plants.

Washington said North Korea would stay on the State Sponsors of Terrorism list until it agreed to a protocol that could verify a nuclear program declared by Pyongyang in June ahead of dismantlement of its atomic arsenal.

'The United States will not take North Korea off the state sponsor of terrorism list until we have a protocol in place to verify the dismantling and accounting for Korea's nuclear program,' said White House spokesman Tony Fratto.

The State Department said Pyongyang's decision to stop disabling its key Yongbyon nuclear complex was of 'great concern' and 'a step backward' in diplomatic efforts aimed at denuclearising the Korean peninsula.

'It certainly is in violation of its commitments to the six-party framework, certainly in violation of the principle of action-for-action,' department spokesman Robert Wood told reporters.

Pyongyang said on Tuesday that it had stopped disabling its nuclear plants at the Yongbyon complex as of August 14 and, instead, wanted to restore them because the United States had failed to remove it from the terrorism blacklist.

The North Koreans claim they should be removed from the blacklist first as part of an action-for-action plan under the six-party accord among China, the United States, the two Koreas, Japan and Russia.

Mr Wood said that Washington would consult with the other parties 'to see how things play out in the coming weeks.' North Korea, which tested an atomic bomb on October 2006, has been negotiating over the last five years under the six party framework on disbanding its atomic program in return for energy aid and diplomatic and security guarantees.

It has already shut down the Yongbyon nuclear reactor and has disabled about 80 per cent of the main nuclear complex.

Experts said Pyongyang's latest decision to stop disabling the nuclear plants was aimed at pressuring US President George W. Bush's administration to take it off the terror list.

With a Middle East peace settlement virtually out of sight and diplomatic efforts to isolate Iran not bearing fruit, the administration has been banking on denuclearisation of the Korean peninsula as a rare foreign policy victory before Bush leaves office in January.

'At this stage, if the North Koreans are not prepared to make specific commitments on verification, I think the process will bog down, probably for the remainder of the Bush administration,' said Mr Robert Einhorn, a former top non-proliferation expert at the State Department.

The North Koreans had hoped that by threatening to reverse the nuclear disablement process, Washington would cave in and make a concession by taking them off the blacklist, he said.

'I don't think the Bush administration will cave in on this,' said Mr Einhorn, now at the Washington-based Centre for Strategic and International Studies.

He also did not believe the North Koreans would carry out their threat to reverse the disablement of the Yongbyon reactor, which Pyongyang itself wanted to shut down because it had approached the end of its normal lifetime.

'I don't think it is a threat they really want to carry out,' Mr Einhorn said.

An agreement on verifying North Korea's nuclear program plan would have paved the way for dismantling North Korea's atomic network as well as the surrender of atomic material and weapons.

US and North Korean officials held talks in New York last week but failed to break the impasse.

Washington reportedly wants to conduct sampling of nuclear materials, make unannounced visits, and inspect unreported facilities in the reclusive North Korea, as part of a proposed verification protocol.

'The US is gravely mistaken if it thinks it can make a house search in the DPRK (North Korea) as it pleases just as it did in Iraq,' said a statement from a North Korean foreign ministry spokesman carried by the official Korean Central News Agency.

Japan and South Korea expressed regret over Pyongyang's move, while China, North Korea's closest ally, made a general statement calling on parties to honour their commitments. -- AFP