North Korea is cancelling a non-aggression pact with South Korea and has restated previous threats following a United Nations Security Council vote to impose more sanctions for its third nuclear test.
The Pyongyang regime's statement was a response to tough new sanctions targeting the country's economy and leadership.
North Korea has already threatened a pre-emptive nuclear strike on the United States.
Pyongyang said it would retaliate with "crushing strikes" if enemies intrude into its territory. It also says it is voiding past nuclear disarmament statements between North and South Korea.
It previously said it was cancelling a hotline with the US and the armistice which ended the Korean War in 1953.