The 12-day manoeuvres, described as among the biggest ever conducted by Washington and Seoul, involve 14,000 US soldiers stationed in South Korea and 5,400 others brought from the US mainland, Japan and Guam, along with 56,000 South Korean troops.

North Korea warned that the exercises could set off a war on the Korean peninsula and adversely affect talks between Pyongyang and Washington.

Pyongyang's official media condemned the exercises as "intentional and premeditated war moves to aggravate the situation on the Korean peninsula and start a war against the North any moment".

But the US rejected North Korea's accusations as unfounded. Lee Ferguson, spokeswoman of the US military command in Seoul said the annual drill was "no more than a routine, defensive training exercise".

The exercise, code-named Ulji Focus Lens, involves mainly computer simulations designed to evaluate and improve joint contingency operations.

The USS Blue Ridge, the flagship of the US 7th Fleet, will lead an unspecified number of warships during the exercise.

The US has warned against any missile launch

The US and South Korea said in an earlier joint statement: "The Combined Forces Command's preparedness is an effective deterrent to external aggression, and serves as the foundation for diplomatic efforts to achieve peace and stability on the Korean peninsula through dialogue."

The exercises come at a time of growing tension in the region over North Korea's suspected plans to test-fire a new Taepodong II long-range missile which could hit parts of the US.

North Korea shocked the region last year when it launched a medium-range Taepodong I missile over Japan and into the Pacific without prior warning.

In missile talks last week, the US warned the North that any launch could shatter the region's stability.

'Anti-reunification moves'

Aside from the joint war games, South Korea has announced a programme of field exercises, including air-raid drills on the ground and anti-submarine drills at sea.

North Korea's official newspaper Rodong Sinmun accused South Korea of mobilising some 200,000 civilians, troops and police force.

It said the exercises were "timed to coincide with their anti-[North Korea] campaign over its non-existent 'missile threat'".

Rodong Sinmun denounced Seoul for "unpardonable anti-national, anti-reunification moves to throw a wet blanket over the nation's desire for reunification".