The United States and South Korea have announced joint naval war exercises as the death toll from North Korea's shelling of a border island rose to four. It was one of the North's worst attacks in decades.
Coastguards have found the bodies of two elderly men on the island of Yeonpyeong, a day after an artillery barrage by the North killed two marines and wounded 18 other people.
Hundreds fled the island on Tuesday, taking ferries to the mainland.
The White House says President Barack Obama telephoned President Myung-Bak Lee to declare that the United States stands shoulder to shoulder with South Korea, where 28,500 American troops are based.
The two countries are due to hold joint military drills beginning on Sunday.
The BBC reports that North Korea claims that Seoul provoked the attack by firing into the North's territory.
In a statement, the North accused Seoul of driving the peninsula to the "brink of war" with "reckless military provocation" and by postponing humanitarian aid, according to the official KCNA news agency in the North.
A US aircraft carrier with 75 warplanes left a naval base south of Tokyo on Wednesday, bound for Korean waters. The USS George Washington has a crew of more than 6000.
China has urged both North and South Korea to show calm and restraint and engage in talks as quickly as possible to avoid an escalation of tensions .