The bodies of two civilians have been found on the South Korean island of Yeonpyeong, shelled in Tuesday's attack by the North.
Two South Korean marines also died, and other people were injured when dozens of artillery shells hit the island - most of them striking a military base.
UN Secretary General Ban Ki-moon called it one of the "gravest incidents" since the end of the Korean War.
The US president has pledged to stand shoulder to shoulder with South Korea.
Military drills
The bodies of two men in their sixties were found on Wednesday. According to the South Korean Coast Guard they were believed to be construction workers.
The latest TV pictures of the island show neighbourhoods reduced to rubble with shops and homes burnt and destroyed.
In a phone call, the US President Barack Obama assured the South Korean president of his country's firm and full commitment to the defence of an ally.
The US and South Korea are to hold joint military drills beginning on Sunday.
A US aircraft carrier with 75 warplanes left a naval base south of Tokyo on Wednesday, bound for Korean waters.
The nuclear-powered USS George Washington has a crew of more than 6,000. The US has a total of some 28,000 troops stationed in the South.
US Forces Korea said the exercise was defensive in nature.
"While planned well before yesterday's unprovoked artillery attack, it demonstrates the strength of the South Korea-US alliance and our commitment to regional stability through deterrence," it said in a statement.
But the BBC's John Sudworth, in Seoul, said the deterrent effect may be limited - the carrier was in Korean waters just a few months ago as part of a show of strength that appears to have done little to temper North Korea's actions.
