A South Korean warship with 104 people on board sank near the North Korean border late Friday after an unexplained explosion, military officials said, as one report said about 40 sailors were missing.
The Seoul government called an emergency security meeting but military officials said there was no indication so far that the North was involved.
Yonhap news agency, quoting unidentified military officials, said about 40 people were feared missing after the blast tore a hole in the hull of the 1,200-ton corvette Cheonan.
The military said earlier that 58 people had been rescued and gave no update on that figure early Saturday.
A spokesman for the Joint Chiefs of Staff (JCS) said he could not confirm the Yonhap report, but indicated that many crewmen were missing. He said official casualty figures would be announced later Saturday.
The news agency said a navy team would dive after daybreak to investigate the cause of the sinking and possibly retrieve bodies.
The Cheonan sank late in the evening near Baengnyeong island in the Yellow Sea, located between the Korean peninsula and China to the west.
Media reports said six naval ships, two coastguard vessels and four other craft were involved in the rescue operation. In Washington, a US defence official confirmed that the cause of the explosion was unclear and said some US navy ships were in the area, standing by to help.
“Rescue operations are still under way,” the JCS spokesman said. “There is no clue yet on any North Korean involvement.”
The JCS said there were no abnormal military movements on the North Korean side of the disputed maritime border, the scene of deadly naval clashes in 1999 and 2002.
Several bodies were retrieved, Yonhap said, without giving an exact figure. Seven crewmen were airlifted to hospital.
“For now, it is not certain whether North Korea is related” to the incident, presidential spokeswoman Kim Eun-Hye said, adding President Lee Myung-Bak had ordered maximum efforts to rescue the crewmen.
“Finding the truth (behind the incident) is important, but saving our sailors is more important,” the president was quoted as saying.
Another emergency meeting was to be held Saturday morning.
A government source quoted by local media said officials were investigating various possible causes: an attack by a North Korean torpedo boat, a mine or an explosion of munitions aboard the ship.
The ship was well south of the disputed border and North Korean ships were unlikely to be in the area, a presidential official said.
Last November the navies of the two nations exchanged fire in the area. Seoul’s officials said a North Korean patrol boat had retreated in flames but its casualties were unknown. No South Koreans were hurt.
The North refuses to accept the maritime border known as the Northern Limit Line, which was drawn up by United Nations forces after the 1950-53 Korean war. It says the line should run further to the south.
In January the North fired 370 artillery shells into the sea near the border, raising tensions between the two sides.
In an indication of sensitivities around the peninsula, Taiwan to the south activated its national security mechanism and President Ma Ying-jeou ordered close monitoring of the situation after Friday’s sinking, the island’s official Central News Agency reported.