South Korean officials say they will take the case of its sunken naval vessel to the U.N. Security Council.
Presidential spokesman Lee Dong-kwan said Sunday that President Lee Myung-bak will deliver a speech about the incident Monday to outline measures Seoul will take against the North, including taking it to the Security Council.
The spokesman said after Mr. Lee delivers his address, his defense, foreign and unification ministers will hold a joint news conference to disclose what steps they will take against the North.
Tensions have been growing between South and North Korea since Seoul accused its communist rival of firing a torpedo at the Cheonan in March, killing 46 sailors.
On Friday, South Korean President Lee called the incident "a military provocation," and said it was a breach of the 1953 armistice that ended the Korean War.
North Korea said Saturday it wants its own investigators to review the evidence, and accused South Korea of faking the incident.
Pyongyang also denounced a separate investigation getting started by the U.N. Command. The command oversees the 57-year-old armistice between the two countries. It announced Friday it will form a special team to investigate South Korea's claims.
North Korea threatened Friday to cut off all ties with South Korea and scrap a bilateral non-aggression pact.
In Tokyo Friday, U.S. Secretary of State Hillary Clinton said the United States is working with Japan, China and South Korea to determine an "international response" to North Korea.
U.S. officials have refrained from calling the attack either an act of war or state-sponsored terrorism.
The United States has about 28,000 troops on the Korean peninsula. U.S. Chairman of the Joint Chiefs of Staff Admiral Mike Mullen said U.S. forces remain at a normal level of readiness.
The area where the South Korean ship sank is near a maritime border designated by the U.N. Command at the signing of the 1953 Korean armistice. Pyongyang has never accepted the border as valid. The two Koreas have fought three naval clashes in the area since 1999.
Some information for this report was provided by AP and Reuters.