The US secretary of state says her country is working hard to avoid an escalation after a report blamed North Korea for the sinking of a South Korean warship.
After talks in China, Hillary Clinton urged countries in the region to contain "the highly precarious situation created by North Korea".
Earlier, South Korean President Lee Myung-bak froze trade with Pyongyang, vowing to punish those who carried out the attack, which killed 46 sailors.
Meanwhile, the Pentagon announced the US was planning to start joint anti-submarine and other naval exercises with South Korea "in the near future".
Pentagon spokesman Bryan Whitman said the decision to start the exercises was "a result of the findings of this recent incident".
'Unequivocal support'North Korea has said it would retaliate for any action taken against it.
The country's main newspaper called the investigation an "intolerable, grave provocation".
President Lee addressed the nation on television to announce Seoul would no longer tolerate "any provocative act by the North and will maintain a principle of proactive deterrence". Link
