North Korea threatened nuclear attacks on Seoul and Washington as South Korea and the US began annual military drills on Monday.
The extreme rhetoric from Pyongyang comes at a time of high tension following the defection of a senior North Korean diplomat and a US plan to place a high-tech defence missile system in South Korea.
The North's military said that it will turn Seoul and Washington into "a heap of ashes through a Korean-style pre-emptive nuclear strike" if they show any signs of aggression toward the North's territory.
The North's "first-strike" units are ready to mount retaliatory attacks on South Korean and US forces involved in the drills according to the warning, carried by Pyongyang's state media.
South Korea's Unification Ministry expressed strong regret over the North's warning, saying the drills with the US are defensive in nature. Seoul and Washington have repeatedly said they have no intention of invading Pyongyang.
This year's Ulchi Freedom Guardian drills began on Monday for a 12-day run and are largely computer-simulated war games. The training involves 25,000 American troops and 50,000 South Korean soldiers.
The drills come just days after Seoul announced that Thae Yong Ho, No 2 at the North's embassy in London, had recently defected to South Korea because he was disillusioned with the North's leadership.
Pyongyang's state media called him "human scum" and a criminal who had been ordered home for a series of alleged criminal acts.
South Korea's president said on Monday there were signs of "serious cracks" in the North's ruling elite class, and that Pyongyang could carry out some action to divert public attention away from such domestic problems.
Many analysts said the defection was an embarrassment to the North Korean government of leader Kim Jong Un, but would not weaken the unity of the country's elite class.
Previous South Korea-US military drills have brought threats of war.
North Korea has already boosted such war rhetoric because of the planned deployment of the US Terminal High-Altitude Area Defence system in South Korea, which Washington and Seoul says is needed because of the increasing North Korean threats.