US Secretary of State Hillary Clinton warned North Korea on Wednesday to halt its "provocations and policy of threats" and said the world must respond to its sinking of a South Korean warship.
Clinton, who is visiting Seoul to show solidarity amid rising inter-Korean tensions, also said the United States was reviewing additional options to hold the North accountable.
The confrontation began when a multinational investigation concluded last week that a North Korean submarine had torpedoed one of Seoul's corvettes on March 26 leading to the loss of 46 lives.
The South, on Monday, announced a package of reprisals, including a halt to most trade. It is also mounting a diplomatic drive to punish the North through the United Nations Security Council.
The North, which denies involvement, announced on Tuesday, that it is cutting all ties with its neighbour. Clinton, at a press conference, said the US had a "rock-solid commitment" to the security of its decades-old ally South Korea.
The Secretary of State, who earlier met President Lee Myung-Bak, praised his "strong and dignified speech", delivered on Monday, in which he announced reprisals. The sinking requires a "strong but measured response", she said.
"We cannot turn a blind eye to belligerence and provocation," she said. "We will stand with you in this difficult hour and will stand with you always," she added.
Clinton backed moves to take the attack to the Security Council, saying the international community has "a responsibility and duty to respond".
She said Washington would consider enhancing its defence posture to deter future attacks. "The United States is also reviewing additional options and authorities to hold North Korea and its leaders accountable," she said without elaborating. "We call on North Korea to halt its provocations and its policy of threats and belligerence towards its neighbours," she announced.