US Secretary of State Hillary Clinton says the international community must respond in the growing crisis over the sinking of a South Korean warship.
She said there was "overwhelming" evidence that North Korea was to blame, and urged Pyongyang to halt its "policy of belligerence".
Mrs Clinton was speaking in South Korea at the end of an Asian tour, the BBC reports.
North Korea denies it was responsible, and has warned of retaliation if action is taken against it.
After an international investigation produced proof that the Cheonan was hit by a North Korean torpedo, South Korea announced a package of measures, including a halt to most trade. It is also seeking action via the United Nations Security Council.
The North then announced on Tuesday that it was cutting all ties with the South. It has also banned South Korean ships and planes from its territory.
South Korean Foreign Minister Yu Myung-Hwan told a joint news conference he and Mrs Clinton had agreed that North Korea should take responsibility for the sinking of the Cheonan, torpedoed on 26 March. Forty-six sailors died.
North Korea's official KCNA news agency says the government will sever all relations with the South Korean government and abrogate an agreement on non-aggression.
The North says it will also expel personnel from the Kaesong industrial park, a joint North-South venture inside the border.
