About 46 crew were missing after a South Korean Navy ship sank near the maritime border with North Korea in the Yellow Sea, media reports in Seoul said Saturday, dpa reported.
Officials were reluctant to connect North Korea to the event, a day after Pyongyang threatened nuclear strikes against South Korea and the United States.
"Finding the truth is important, but it's more important to save our personnel," President Lee Myung Bak said in an emergency meeting of security-related ministers late Friday, his spokeswoman Kim Eun Hye said. Another meeting is to follow Saturday.
The 1,200-ton naval ship Cheonan, carrying 104 crew members on patrol, began taking on water around 9 pm (1200 GMT) Friday near the island of Baengnyeong off the west coast of South Korea, the presidential office said.
An explosion reportedly ripped a hole into the ship's rear hull, leading to engine and power failure, followed by the ship's quick demise, Yonhap News Agency said, quoting unidentified government officials. A number of crew members jumped into the water, and several are dead, Yonhap said.
Military officials could not confirm any deaths, but said two had been airlifted for emergency medical care.
"An unidentified reason caused a hole in the ship, which led to its sinking. Currently 58 have been rescued out of the total 104 on board. Rescue efforts are under way," South Korea's Defence Ministry said.
"It has not been determined whether this incident is related to North Korea," Kim from the president office, The Blue House, said in a briefing Saturday morning.
The Joint Chiefs of Staff, represented by Rear Admiral Lee Ki Shik, also told reporters, "We are very cautious about pointing fingers at North Korea or any other causes at the moment. We will take prompt measures as soon as the cause is revealed."
Local residents' reports of hearing "loud artillery fire" sparked fears of a North Korean attack Friday night around 11 pm (1400 GMT), local media said.
"The loud firing sound lasted for about 15 minutes while I watched TV. In all my time on Baengnyeong island I've never heard such loud sounds of fire, and it was definitely different from what I've heard from usual drills," said one resident quoted by Yonhap.
But the Joint Chiefs of Staff explained the commotion as a South Korean naval vessel's fire on an unidentified object, later said to be a flock of birds, not a hostile ship.
The ship was well south of the Northern Limit Line, the maritime border that the North refuses to recognize, and satellite pictures and other information later showed no sign of the North Korean military, said local media, quoting a presidential official.
Officials had been investigating various possibilities, including a North Korean torpedo or mine attack or the detonation of explosives aboard the Cheonan, a government source told Yonhap.
The Northern Limit Line was the site of bloody naval skirmishes in 1999, 2002 and 2009.