About 40 sailors are missing after a South Korean navy ship sank near the border with North Korea. Yonhap news agency citing military officials said that the patrol vessel with 104 people aboard sank after an unexplained explosion tore through its hull. Several sailors also died as divers prepared to return to the scene after daybreak. South Korean officials played down earlier reports that it may have been the result of an attack by North Korea. There was no sign of the North's military in the area where the ship sank. South Korean President Lee Myung-bak who had convened an emergency meeting of security officials, had ordered the military to focus on rescuing the sailors. The police force has been put on high alert in the capital, Seoul. A South Korean presidential spokeswoman said it was premature to say what caused the Cheonan to sink.