Almost 50 sailors were missing after a South Korean warship sank near the tense border with North Korea, the military said Saturday, but officials said there was no sign so far the North was to blame.

President Lee Myung-Bak called an emergency security meeting and ordered a swift and thorough probe into what appeared to be one of the country's worst military tragedies for decades.

The Joint Chief of Staff (JCS) said 58 sailors had been rescued but 46 others were still missing as of mid-afternoon on Saturday.

"Many of the missing people might have been trapped inside the sunken ship," JCS spokesman Lee Ki-Sik told a parliament committee.

A team of 18 navy divers had started an underwater examination to investigate the cause of the sinking in the Yellow Sea, another JCS official said.

The 1,200-tonne corvette sank Friday evening near Baengnyeong island after a still unexplained explosion. The craft turned turtle, with its hull protruding from the shallow but near-freezing waters.

Reports said the 88-metre (290-foot) craft would have been carrying French-made Exocet and US-made Harpoon anti-ship missiles as well as torpedos and other weaponry.

The military said 13 of the 58 known survivors were injured but in stable condition. Navy ships and surveillance planes were searching for more survivors.

The military said there were no abnormal military movements at the time on the North Korean side of the disputed maritime border, the scene of deadly naval clashes in 1999 and 2002.

"We are detecting no abnormal movement from North Korea," JCS spokesman Park Sung-Woo said.

JCS officer Lee Ki-Shik told journalists the military was "very cautious about pointing fingers at North Korea or any other causes at the moment."

Baek Seung-Joo, an analyst at the defence ministry's Korea Institute for Defence Analyses, said the government appeared to suspect an accident rather than sabotage.

President Lee, who summoned an emergency security meeting immediately after the sinking, called two more sessions on Saturday.

He ordered a "thorough and swift probe" into the cause of the sinking, "keeping all windows of possibility open," a spokeswoman said.

Yonhap news agency quoted a military source as saying the force of the blast near the propeller had lifted the vessel about a foot (30 centimetres) into the air.

Military officials said they had no information to confirm the report.

Some US navy ships were in the area, standing by to help if necessary. The US stations 28,500 troops in South Korea.

In Washington, the State Department said it had no evidence of North Korean involvement.

Last November the navies of the two Koreas exchanged fire in the area and a North Korean patrol boat retreated in flames with unknown casualties.

The two Koreas have remained technically at war since their 1950-1953 conflict ended only in an armistice.

The North refuses to accept the maritime border known as the Northern Limit Line, which was drawn up by United Nations forces after the 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.