The border area between the two countries remains heavily fortified
Communist North Korea has said it is scrapping all military and political agreements signed with the South, accusing Seoul of hostile intent.
South Korea's government had pushed relations "to the brink of a war", the North's cross-border relations body said on state media.
Seoul expressed regret at the move, while the US called it "unhelpful".
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.
TROUBLESOME SEA BORDER
Known as Northern Limit Line
Position in Yellow Sea declared by UN in 1953
Not recognised by North
Deadly naval skirmishes along the line in 1999 and 2002
Regularly breached by North's fishermen
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 programme.
Earlier this week, North Korea criticised 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's John Sudworth 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.
HAVE YOUR SAY
North Korea is an isolated state and when it isn't getting enough attention acts like a petulant child.
A more pessimistic analysis suggests that the rising tension does raise the possibility of small-scale military clashes, says our correspondent.
"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.
Dec 07: Lee Myung-bak wins South Korean presidential election. Vows tougher line on the North
March 08: North expels S Koreans from joint industrial park after Seoul says it will link its aid more closely to the nuclear disarmament issue
April 08: N Korean media warns President Lee his tough stance could have "catastrophic consequences"
July 08: Pyongyang rejects President Lee's offer of direct talks
Oct 08: Military officials from both sides hold first direct talks since President Lee took office
Nov 08: N Korea says it will close land borders, suspend tourism trips and the joint train service because of "relentless confrontation" from Seoul
Dec 08: N Korea enforces stricter border controls and expels hundreds of South Koreans from the joint industrial zone
A spokesman for the US state department, Robert Wood, said the North's rhetoric "is distinctly not helpful, but that's not going to deter us from continuing our efforts to achieve denuclearisation of the Korean peninsula" through long-running talks between the Koreas, regional powers and the US.
The two Korean 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 Demilitarised 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 Mr 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 Mr Lee whom it calls a "traitor".
"Never to be condoned are the crimes the Lee group has committed against the nation and reunification by bedevilling overnight the inter-Korean relations that had favourably developed amidst the support and encouragement of all the Koreans and ruthlessly scrapping the inter-Korean agreements," the North said on Friday.