Tens of thousands of US and South Korean troops Monday began an annual military exercise despite threats from North Korea, which claims the drill is a preparation for nuclear war.

The North's supreme military command announced it has placed its 1.2 million-strong armed forces on alert in response to the start of what it called "test nuclear war manoeuvres".

The ten-day Key Resolve/Foal Eagle drill involves 18,000 US troops, including 10,000 not based in Korea, and 20,000 South Korean troops, said a spokesman for the US-South Korean Combined Forces Command.

Numbers are smaller than last year and no US aircraft carrier will be involved, but the spokesman said the size of the drill was guided only by operational reasons.

South Korea and the United States, which stations 28,500 troops in the country, say the exercise is purely defensive.

The North habitually blasts it as a prelude to invasion and threatens counter-measures, although the war games normally pass off without major incident.

The communist state's military Sunday threatened "merciless physical force" in response to any attack and said it was no longer bound by the armistice that ended the 1950-53 Korean War.

It vowed to halt nuclear disarmament efforts and strengthen its atomic arsenal.

The North is under pressure to return to six-nation nuclear disarmament talks which it abandoned last April, a month before staging a second nuclear test.

On Monday the supreme command of the military, which is headed by national leader Kim Jong-Il, said troops have been alerted to repel any attack and "blow up the citadel of the aggressors" if ordered.

It called for regulars and reservists to undergo training "to mercilessly crush the aggressors should they intrude into the inviolable sky, land and sea of the DPRK (North Korea) even 0.001 mm".

South Korean officials said no unusual military movements had been reported in the North and border crossings were going ahead normally.

During last year's exercise the North three times cut off access to a jointly-run industrial park at Kaesong just north of the border.

Watched by riot police, some 20 activists rallied outside a US military base used as an exercise command post at Seongnam in Seoul's southern outskirts.

"Stop exercise aimed at attacking North Korea!" they shouted.