"The joint agreement to keep the Korean Peninsula nuclear free was nullified because of a sinister and hostile U.S. policy against North Korea," the North's official news agency KCNA said in a statement.
The statement was monitored by South Korean news agency Yonhap.
The two Koreas signed the agreement in January 1992 pledging to renounce hostilities and ban the development and deployment of nuclear weapons on the divided peninsula.
It was the last remaining legal obligation under which North Korea was banned from developing atomic arms. In January this year, Pyongyang withdrew from the Nuclear Nonproliferation Treaty, a global accord to prevent a spread of nuclear weapons.
Nullifying the treaty was the latest move in a months-long dispute with the United States and other countries over the North's nuclear ambitions.
Tension over the nuclear dispute spiked last month during the Beijing talks, when, according to U.S. officials, North Korea claimed to have nuclear weapons and threatened to use or export them, depending on U.S. actions.
During the talks last month, U.S. officials said North Korea also claimed it had reprocessed 8,000 spent nuclear fuel rods — a move that could yield several atomic bombs within months.
However, U.S. and South Korean officials said they could not verify the claim and suggested North Korea may be bluffing in an attempt to increase its leverage in talks with the United States over its suspected nuclear weapons programs.
The United States recently gave South Korea a satellite photograph showing smoke coming from a North Korean nuclear facility, a possible sign the communist nation has started reprocessing spent fuel rods, a South Korean official said Thursday.
Reprocessing the rods would be a key step toward producing nuclear weapons.
The official, who spoke on condition of anonymity, said other signs of nuclear activity, such as traces of chemicals used in reprocessing or heat signatures, had not been detected from the Yongbyon nuclear complex. He said the smoke was coming from radiation and chemical laboratories in the facility.
A spokesman for South Korea's Unification Ministry, Kim Jung-ro, said Thursday that the renewed activity at Yongbyon did not necessarily mean the North was reprocessing.
"We are not sure if they are doing it as an extension of the bluffing or if it is a step to develop nuclear weapons. We need more evidence," he said.
The meetings in Beijing last month, which involved Chinese officials, represented a compromise. North Korea has pressed for bilateral talks, while the U.S. favors a multilateral approach.
The talks were the first since the crisis flared in October, when Washington said North Korea admitted running a secret nuclear weapons program in violation of a 1994 treaty.
That prompted the United States to cut off fuel supplies that it was shipping to the North under the 1994 deal. North Korea, which asserted that the U.S. had already violated the deal by failing to officially recognize it, kicked out nuclear inspectors from its Yongbyon plant.
Pyongyang claims it is justified in building up its armaments because of a hardening of U.S. policy.
Since taking office, President Bush has listed North Korea as a member of the "axis of evil," articulated a policy of preemptive strikes against enemies and approved a missile defense system aimed chiefly at nullifying North Korea's threat to the U.S. mainland.