South Korea will refer Cheonan issue to the United Nations Security Council (UNSC) within the day, Seoul's Yonhap News Agency said on Friday.

South Korean President Lee Myung-bak will specify the issue when he delivers a keynote speech in a regional security forum in Singapore, Yonhap said.

The 1,200-ton Navy frigate "Cheonan", with 104 crew members onboard, went down on March 26 off the South Korean island of Baekryeong off the west coast due to an unexplained explosion that split it in two. Forty-six sailors were killed.

The government formed a joint probe team involving civilian, military experts and foreign experts from the United States, Australia, Britain and Sweden, to investigate the incident.

South Korea on May 20 announced the outcome of investigations over the Cheonan incident, which said the warship that went into waters near a tense maritime border with the Democratic People's Republic of Korea (DPRK) in March was "torpedoed by the DPRK."

The DPRK immediately rejected Seoul's claims that it sank the warship by a "submarine", and said it will dispatch inspectors to South Korea to verify Seoul's claim. But Seoul rejected Pyongyang' s demand.