The United States and South Korea began military drills Monday despite North Korea's threat of nuclear retaliation. North Korea called the drills an invasion rehearsal, according to CNN.
A spokesman for North Korea's military was quoted by the country's state-run media saying it will "turn the stronghold of provocation into a heap of ashes through Korean-style pre-emptive nuclear strike" if the U.S. and South Korea "show the slightest sign of aggression."
South Korea's Unification Ministry said the drills are purely defensive in nature and that they have no intentions of invading Pyongyang.
About 25,000 U.S. troops participate in the annual drill, which is an exercise to "enhance Alliance readiness, protect the region and maintain stability on the Korean peninsula."
The Ulchi Freedom Guardian drills primarily consist of computer-simulated war games.
Word of the North Korean threat comes just days after a senior North Korean diplomat, Thae Yong Ho, defected to South Korea.
Pyongyang's state media responded calling him "human scum" and accusing him of criminal acts including sexually assaulting a minor.
The Associated Press contributed to this report.