North Korea has threatened to revoke an armistice in retaliation for what it claimed were "confrontation moves" by South Korea and the United States, which are holding joint military exercises in the region, the KCNA news agency reported Tuesday.
Pyongyang had earlier warned US forces in South Korea of "miserable destruction" if the two countries were to go ahead with the two-month manoeuvres, called Foal Eagle.
North Korea reiterated its accusation at the time that the South and its US ally were rehearsing for an invasion.
North Korea "will achieve nothing by threats or provocations, which will only further isolate North Korea," White House spokesman Jay Carney said.
The UN Security Council was weighing further sanctions against Pyongyang for it nuclear activities, and Carney called the draft resolution a "credible and strong response that further impedes the DPRK's nuclear activities."
Pyongyang's third known nuclear test on February 12 drew international condemnation, including from the UN Security Council and China.
The North's Korean Central News Agency quoted army officer Ri Chol Hak as saying Tuesday: "Foal Eagle is a drill for invading ... and a drill for a nuclear war."
The United States sided with South Korea against the North in the 1950-53 Korean War, and Washington's and Pyongyang's relations have been marked by antagonism ever since. They have never established formal diplomatic ties.
South Korea remains technically at war with the North after a ceasefire ended the Korean War. A full peace treaty was never signed.