South Korean officials are not ruling out the possibility of a North Korean attack in the sinking of one of the South's naval vessels. About half the more than 100 people on board the ship were rescued, and search efforts are still under way for the remaining crew. An explosion damaged the ship in a historically tense disputed border area.

South Korean President Lee Myung-bak ordered an emergency meeting of senior security officials Friday night, hours after an apparent explosion caused a South Korean patrol boat to sink.

Lee Gi-sik, with South Korea's Joint Chiefs of Staff, told reporters the military's primary focus was on saving those aboard.

He says at 9:45pm local time, the South's ship was passing southwest of South Korea's Baekryoung island when it began sinking due to damage on its bottom side. He says other vessels were mobilized and dozens of people were rescued from the distressed ship.

Baekryoung island is located in waters west of the Korean peninsula, very close to what is called the Northern Limit Line. The maritime border was designated by the United Nations at the signing of a 1953 armistice to halt fighting in the Korean War.

North Korea has never accepted the border as valid, and the North and South have fought at least three separate naval clashes in the area in the past 11 years.

South Korean officials are not yet pointing any fingers but say they are not ruling out the possibility that the vessel was the target of a North Korean torpedo attack.

Lee says a South Korean naval vessel detected an unidentified object on its radar and fired several warning shots. He says he cannot confirm what the object was.

North Korea warned this month it was bolstering its defenses in response to routine annual military drills South Korea conducts with the United States.