North Korea has threatened to scrap the truce that ended the Korean War in 1953.
The announcement was made in anticipation of a United Nations Security Council resolution to be put forward by the United States and China supporting tougher sanctions against the nation for its third nuclear test on 12 February.
The North's military said it could launch a "precise" strike anytime, unrestrained by the armistice. It also warned it could mount a strike with atomic weapons to counter any US nuclear threat.
The North said the armistice that ended the 1950-53 war will be "completely" nullified from 11 March, when a South Korean-US exercise gets into full swing in the South.
The ABC reports an annual exercise known as Foal Eagle began on 1 March and will run until 30 April, involving more than 10,000 US troops and a far greater number of South Korean personnel.
In a statement on official media, the North Korean military called the joint exercise a "most blatant" provocation.
The Korean armistice was never followed by a peace treaty and the combatants in the conflict technically remain at war.