South Korea bans trade with North as Obama gives support

South Korea's president lashed out at North Korea by suspending trade as retaliation for a torpedo attack on a warship that killed 46 soldiers.

President Lee Myung-bak also banned the North's cargo ships from passing through its waters. "North Korea will pay a price corresponding to its provocative acts," he said.

The White House said that President Barack Obama "fully supports" the South's action.

The administration said it endorsed Mr Lee's demand that "North Korea immediately apologise and punish those responsible for the attack, and, most importantly, stop its belligerent and threatening behaviour".

"US support for South Korea's defence is unequivocal, and the president has directed his military commanders to coordinate closely with their Republic of Korea counterparts to ensure readiness and to deter future aggression," the White House said.

The South's president also vowed to take the North to the UN Security Council for punishment over the sinking of the warship Cheonan. The attack was South Korea's worst military disaster since the Korean War.

US Secretary of State Hillary Clinton said the North must be held accountable. She is pushing to get the support of China, North Korea's ally, for diplomatic action. "We are working hard to avoid an escalation of belligerence and provocation," she said.

A team of international investigators concluded last week that a North Korean submarine launched the torpedo that sank the Cheonan.