South Korea will take the case of its sunken naval vessel Cheonan to the UN Security Council, as pressure mounts on North Korea, which is accused of torpedoing the ship.
Lee Myung-bak, the South Korean president, will deliver a speech about the incident on Monday, Lee Dong-kwan, the presidential spokesman, said.
"The president will present frameworks of measures, one about our own steps and the other about measures through international co-operation ... He will also mention a plan to bring the case to the UN Security Council," Lee said on Sunday.
Another senior government official told Yonhap News Agency: "We are focusing on diplomatic means of punishment as we found out that existing sanctions and resolutions are not enough. It is time to decide whether we go over to another step."
Last week, Seoul released the findings of a report which concluded that a North Korean submarine had fired a torpedothat sank the Cheonan corvette, killing 46 sailors.
The North has denied the accusation.
Lee said the president would also demand a response from the North.
"President Lee Myung-bak may mention the name of the leader Kim Jong-il [in the conclusion of the speech]," he said.
Washington has called for an "international response" to the sinking,without specifying what form this might take.
An international response could range from fresh Security Council sanctions on North Korea, although those might be opposed by China, to a statement of condemnation by the world body.
A range of sanctions are already in place against North Korea for its missile and nuclear tests of recent years.
South Korea has repeatedly said it would not strike back at the North, aware that would frighten away investors already weary about the escalating tension on the divided peninsula.
The two Koreas are still technically at war because their 1950-53 conflict
ended with an armistice, not a peace treaty.