Park In-kook said he handed a letter to Mexican Ambassador Claude Heller, the council chairman this month, requesting “action by the Security Council commensurate with the gravity of the situation.”

The South Korean envoy gave no further details on the content of the letter. Heller meanwhile told reporters that he would start consultations with other council members “to give an appropriate reply.”

The United States, the South’s biggest ally, said Seoul may not seek a full Security Council resolution because of rising tensions after the sinking of the navy corvette Cheonan in March.

Seoul said it would hold discussions with its allies to ensure some action was taken.

As North Korea stepped up its hostile rhetoric, the top US military commander in the region said war was unlikely but US forces were prepared for any eventuality.

“Today, the Republic of Korea government referred the matter of North Korea's attack against the Cheonan to the UN Security Council,” South Korean President Lee Myung-bak said at a security conference in Singapore.

“If we are to once again tolerate North Korea’s blatant act of violence, then I believe that will not promote but endanger the peace and stability of the Korean peninsula and that of Northeast Asia,” he said.

Lee added: “North Korea must admit its wrongdoing, it must pledge to never again engage in such reprehensible action. This is in the interest of peace. This is in the interest of North Korea.”

Lee's foreign policy adviser, Chung Min Lee, later told reporters Seoul expected some form of action. “Whether that will result in a new sanction or resolution or a chairman's statement from the Security Council is something we have to work on in the days and weeks ahead,” he said.

South Korea has blamed the North for torpedoing the Cheonan in March, killing 46 sailors. The North denies responsibility and has regularly accused the South of staging the incident to help Lee in local elections this week.

The United States has said it will back South Korea in any proceedings at the Security Council. However, diplomats have said China, a veto-wielding Security Council permanent member and North Korea's only major ally, has made clear that Beijing would prefer not to take up the issue at the United Nations.

China, Western diplomats say, will not tolerate new sanctions against North Korea, while Seoul appears determined to have the council at least agree on some form of rebuke of North Korea.

US Defense Secretary Robert Gates said planned joint US-South Korean military exercises may not take place until it becomes clear what action the United Nations will take.

“It is not clear exactly what path the Republic of Korea intends to pursue in the UN, whether it’s a resolution or a president's letter, whatever,” Gates told reporters in Singapore, where he was also attending the security conference.

“It may be that there's a desire first to see what can be accomplished at the UN and then think about next steps beyond that,” he said, referring to the exercises.