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The US and South Korea on Sunday launched a major naval exercise involving a nuclear-powered US aircraft carrier in the Sea of Japan despite North Korea's threats of nuclear retaliation.
The drill is 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 the South's Defence Minister Kim Tae-Young said in a joint statement this week after talks.
South Korea and the United States, citing findings of a multinational investigation, accuse the North of torpedoing a South Korean warship near the tense Yellow Sea border in March.
The communist North 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 some 200 fixed-wing aircraft.
Some 8,000 service personnel from the two allies were to take part in the show of force.
"The USS George Washington left the southern port of Busan around 7:00am Sunday (2200 GMT Saturday). It's sailing towards the Sea of Japan (East Sea) for the exercise," a US military spokesman told AFP.
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 (West Sea) to the Sea of Japan in deference to Chinese protests.
But future drills would be held in both seas.
North Korea has denounced the exercise as "very dangerous sabre-rattling" and on Saturday threatened to respond with nuclear weapons.
Pyongyang routinely threatens war in response to joint military exercises by the two long-time allies, calling them a rehearsal for war.
"All these war manoeuvres are nothing but outright provocations aimed to stifle the Democratic People's Republic of Korea by force of arms to all intents and purposes," the North's National Defence Commission said in a statement on Pyongyang's official news agency.
"The army and people of the DPRK will legitimately counter with their powerful nuclear deterrence the largest-ever nuclear war exercises to be staged by the US and the South Korean puppet forces."
The exercise "is as reckless an act as waking up a sleeping tiger", it said.
In response, Washington called on North Korea to tone down its "provocative" statements.
"We are not interested in a war of words with North Korea," said State Department spokesman Philip Crowley. "What we need from North Korea is fewer provocative words and more constructive action."
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.
