The war games are the first in a series intended "to send a clear message to North Korea that its aggressive behaviour must stop," US Defence Secretary Robert Gates and South Korean Defence Minister Kim Tae-Young have said.

Seoul and Washington, citing the findings of a multinational investigation, accuse Pyongyang's communist regime of torpedoing a South Korean warship near the tense Yellow Sea border in March.

North Korea denies involvement in the sinking of the Cheonan, which claimed 46 lives.

The US-led United Nations Command said the four-day drill would involve about 20 ships, including the aircraft carrier USS George Washington, and 200 fixed-wing aircraft.

Around 8,000 service personnel from the two allies were to take part in the show of strength.

"The USS George Washington left the southern port of Busan around 7:00 am Sunday (2200 GMT Saturday). It's sailing towards the Sea of Japan (East Sea) for the exercise," a US military spokesman told AFP earlier.

Officials at Seoul's defence ministry said other navy ships had also left Busan and the nearby port of Jinhae for the drill, with some from the US 7th Fleet set to join them off the peninsula's east coast.

The ministry has said the drill had been relocated from the sensitive Yellow Sea to the Sea of Japan, also known as the East Sea, in deference to Chinese protests.

But future drills would be held in both seas.

North Korea, which has routinely criticised US-South Korean drills as a rehearsal for war, denounced the exercise as "very dangerous sabre-rattling" and threatened to respond with nuclear weapons at the weekend.

Minju Joson, a newspaper published on behalf of the North's cabinet, said the drills were being conducted by "the US imperialists and the South Korean puppet warmongers."

The newspaper also repeated a warning of nuclear retaliation made by Pyongyang's top defence body a day earlier.

"The army and people of the DPRK (North Korea) will take strong retaliatory measures with dignity by dint of their powerful nuclear deterrent, as a spokesman for the DPRK National Defence Commission had declared," it said.

"They will start Korean-style sacred war for retaliation any time they deem it necessary."

Washington has urged the North to tone down its "provocative" statements, while China -- Pyongyang's ally -- called for restraint from all sides.

"All involved parties should commit to providing peace and security on the Korean peninsula," China's foreign minister Yang Jiechi told a joint press conference Sunday in Vienna after meeting his Austrian counterpart.

South Korea's military says it is closely monitoring the North's military moves in border areas but had not detected any unusual activities in the lead-up to the exercises.

Yang Moo-Jin, a professor of Seoul's University of North Korean Studies, told AFP that Pyongyang could test-fire long-range missiles or conduct new tests on its diversified nuclear programmes.

"The North may try to either stage a third underground atomic bomb test based on its plutonium-based programme or carry out a nuclear fusion reaction again in a show of force," Yang said.

In May, the North claimed its scientists had carried out a nuclear fusion reaction that could lead to a limitless supply of clean energy. Nuclear fusion can also be employed to make hydrogen bombs.

Pyongyang already said last September that it was in the final stages of experimental uranium enrichment -- a second way of making atomic bombs.

Six-nation talks aimed at dismantling the North's nuclear programmes have been stalled since December 2008.

The North announced it was quitting the forum in April last year and it staged its second atomic weapons test the following month, incurring tougher United Nations sanctions.

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