A sticking point in talks has been how to verify North Korea's disarmament
North Korea says it has stopped disabling its nuclear facilities, accusing the US of reneging on a six-party disarmament deal.
Work was suspended on 14 August, a foreign ministry spokesman told the state news agency KCNA.
North Korea says it took the step because the US failed to remove it from a list of state sponsors of terrorism.
But the US says it wants to agree more stringent verification processes before it does so.
North Korea finally submitted a long-delayed account of its nuclear facilities to the six-party talks in June - and was expecting to be removed from the US list of terrorism sponsors in return.
But that move has been delayed amid wrangling among the six parties - North and South Korea, the US, China, Russia and Japan - over how to verify the North's declaration.
The North has also threatened to restore facilities at its main Yongbyon plant - where the main cooling tower was spectacularly demolished in late June in a symbol of Pyongyang's commitment to disarmament.
The submission by North Korea of a nuclear dossier was seen by the Bush administration as one of its success stories in foreign policy, says the BBC's diplomatic correspondent Jonathan Marcus.
The question now is whether this latest spat is just another hiccup in relations or represents a serious blow to progress.
With only months to go until a new team takes office in Washington, Pyongyang may be looking to the next administration for a better deal, our correspondent says.
'Action for action'
"As the US side failed to keep its own side of the agreement, we cannot but take the following measures under the principle of action for action," the foreign ministry spokesman told the KCNA, according to AFP news agency.
"First, we've decided to stop the denuclearisation process that has been under way in accordance with the 3 October agreement. This measure already took effect on 14 August and relevant parties have already [been] informed.
"Second, we will consider restoring the Yongbyon facilities to their original state in accordance with strong demands from our relevant agencies."
US State Department spokesman Sean McCormack, travelling with Secretary Condoleezza Rice in Jerusalem, said he had no immediate comment on the KCNA report, according to Reuters news agency.
Analysts say that, in addition to the delay removing it from the US terrorism list, the North has been angered by recent military drills jointly undertaken by US and South Korean forces.