The US and South Korea Sunday began major naval manoeuvres in the Yellow Sea, five days after North Korea's attack on a South Korean island escalated tensions in the region.
A spokesman for the general staff of the South Korean military in Seoul confirmed that the exercise had started. The four-day display of air and sea forces is intended to send a strong signal to communist North Korea to back down.
Ten warships are taking part, including the nuclear-powered aircraft carrier USS George Washington.
The manoeuvres are taking place away from the disputed maritime border between South and North Korea, in an area near the coastal city of Taean, about 150 km south of Seoul.
North Korea, which as usual condemned the joint troop operations as a provocation, warned the two countries of unforeseen consequences if the programme went forward.
China expressed concern about the military exercises in the Yellow Sea. US Secretary of State Hillary Rodham Clinton pressed her Chinese counterpart Saturday to "make clear that North Korea's behaviour is unacceptable".
On Tuesday, North Korean artillery attacked South Korea's Yeonpyeong Island near the two countries' disputed maritime border, killing two South Korean soldiers and two civilians. South Korea answered in an hour-long exchange of fire.
Seoul has refuted North Korea's insistence that South Korea fired first. It was the most violent incident since the end of the Korean conflict (1950-53.)