South Korea has referred North Korea to the United Nations over the sinking of its warship in March.
An international investigation found that North Korea was responsible, an accusation the North denies.
South Korea has not spelled out what action it wants the UN to take, although it has not asked for sanctions against the North.
Tensions between the two Koreas have increased since the sinking in which 46 sailors were killed.
In a speech to an Asian security conference in Singapore, South Korean President Lee Myung-Bak said: "North Korea must admit its wrongdoing."
US Defence Secretary Robert Gates, who is attending the conference, said planned joint US-South Korean military exercises may be delayed until the UN decides what action it 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," Mr Gates said.
Earlier he had said that Washington and Seoul were looking into holding additional military manoeuvres in response to the sinking, to reassure Seoul.
But he said North Korea was "more unpredictable than usual".
This is the first time that South Korea has taken North Korea to the UN Security Council for an inter-Korean dispute.
However, the UN is unlikely to back sanctions: North Korea's closest ally, China, has a veto on the Security Council.