The North`s military has 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 extreme rhetoric from Pyongyang comes at a time of high tension. A US plan to place a high-tech defence missile system in South Korea and the defection of a senior North Korean diplomat, Thae Yong Ho, has deeply antagonised the North Korean leadership.

Now, state media in Pyongyang have warned the country`s "first-strike" units are ready to mount retaliatory attacks on South Korean and US forces involved in the drills.

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 manouveres, known as Ulchi Freedom Guardian, 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.

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.

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