South Korea has referred North Korea to the United Nations Security Council over the sinking of a South Korean warship that Seoul says was attacked by a North Korean submarine. The action was announced Friday by South Korean President Lee Myung-bak, during a visit to Singapore.

North Korea has strongly denied any involvement in the March attack, which killed 46 South Korean sailors. Just hours before Lee spoke, Pyongyang threatened what it called the "toughest retaliation" if Seoul takes the matter to the UN Security Council.

Lee did not say if South Korea would push for sanctions against the North, but called the sinking of the Cheonan "reprehensible" and said Pyongyang must admit its wrongdoing.

U.S. Defense Secretary Robert Gates said in Singapore Friday that joint naval exercises with South Korea may be postponed to allow time for Seoul to secure more diplomatic support at the UN. Those exercises had been scheduled for next week.

The U.S. naval commander in the Pacific said earlier Friday that he sees no signs North Korea is planning any provocations or attacks on the South.

Speaking on the sidelines of the security conference in Singapore, Admiral Robert Willard said he had instructed his forces to be especially vigilant but that he had not seen North Korea massing troops along the border or preparing another nuclear test.

The two Koreas signed an armistice in 1953, but technically remain in a state of war.

Some information for this report was provided by AP and Reuters.