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.
Sooner or later North Korea is going to start a war it cannot win.