South Korea's government had pushed relations "to the brink of a war", the North's cross-border relations body said on state media.

South Korea expressed regret at the announcement and called for dialogue.

Relations have deteriorated since South Korea's President Lee Myung-bak took a harder line approach to the North.

One agreement the North said it was to scrap covers the maritime border in the Yellow Sea.

The two countries' navies fought bloody skirmishes in the area of the de facto border in 2002 and 1999.

"All the agreed points concerning the issue of putting an end to the political and military confrontation between the North and the South will be nullified," the Committee for the Peaceful Reunification of Korea said.

It said that the situation on the Korean peninsula had reached a point where there was "neither way to improve [relations] nor hope to bring them on track".

The North has stepped up rhetorical attacks on the administration of South Korean President Lee Myung-bak, who has promised to stop the free flow of aid to the North unless it moves to end its nuclear weapons program.

Earlier this week, North Korea criticized the appointment of a new South Korean unification minister, describing the choice of Hyun In-taek as evidence that the South wanted to intensify confrontation between the two Korean states.

The BBC in Seoul says some analysts believe that Pyongyang is trying to build up tensions with the South in order to give itself more negotiating power with the new US administration.

A more pessimistic analysis suggests that the rising tension does raise the possibility of small-scale military clashes, says our correspondent.

Regrets

"Our government expresses deep regret," said Kim Ho-Nyoun, spokesman for South Korea's unification ministry, which handles cross-border affairs.

"We urge North Korea to accept our call for dialogue as soon as possible," he said.

The two states are still technically at war because their three-year conflict ended in a truce, not a peace treaty, in 1953.

The peninsula remains divided by a heavily fortified Demilitarized Zone, with thousands of troops stationed on both sides of the border.

Relations improved in the past decade, with North Korean leader Kim Jong-il meeting with then-South Korean President Kim Dae-jung in a historic summit in 2000.

But tensions have been high since Lee took office in Seoul nearly a year ago pledging to get tough with Pyongyang.

He began rolling back his predecessors' "sunshine policy" of unconditional aid to the North.

The North responded by cutting off talks, suspending key joint projects and stepping up criticism of Lee who it calls a "traitor".

"Never to be condoned are the crimes the Lee group has committed against the nation and reunification by bedeviling overnight the inter-Korean relations that had favorably developed amidst the support and encouragement of all the Koreans and ruthlessly scrapping the inter-Korean agreements," the North said on Friday.