President Lee Myung-Bak ordered the military to stay alert against North Korea after the sinking Friday night of the 1,200 tonne corvette in the Yellow Sea.

Defence Minister Kim Tae-Young has said a drifting North Korean mine dating back to the 1950-53 war might have caused the blast which left 46 missing, or the North might have intentionally sent a mine floating towards the Cheonan.

Navy chief Kim Sung-Chan, in a report to the president, said the ship's munitions storage room did not appear to have exploded and "the ship was broken in two because of powerful outside pressure or an (exterior) explosion".

Hopes of finding survivors faded Monday when divers heard no response after banging on the two sections of the sunken hull off Baengnyeong island. But angry and tearful relatives have been demanding swifter rescue action.

Dozens of divers have braved the Yellow Sea's strong currents and frigid and murky waters, trying in vain to get inside the hull. One of them, father of two Han Joo-ho, fell unconscious Tuesday and later died, the military said.

Navy Lieutenant Colonel Song Moo-Jin said the divers were working "in a very vicious environment" and could stay underwater for only about 20 minutes at a depth of 40 metres (130 feet).

Lee has cautioned against jumping to conclusions about the cause of the blast but ordered the military Tuesday to remain alert.

The disputed Yellow Sea border was the scene of deadly naval clashes in 1999 and 2002 and of a firefight last November.

"Since the incident happened at the frontline, the government should be thoroughly prepared to cope with any movement on the part of North Korea," Lee told a cabinet meeting in Seoul.

"The armed forces are urged to maintain full alert without the slightest breach," he said, before flying by helicopter to Baengnyeong Island near the border with an escort of jet fighters.

US Deputy Secretary of State James Steinberg said in Washington he had heard nothing to implicate any other country in the tragedy.

Lee was the country's first president to visit Baengnyeong Island, 16 kilometres (10 miles) from North Korea's coast, where guided missile and artillery batteries are deployed.

"Fighter jets were on patrol during his trip to the island because it is very close to major North Korean military camps," said his spokesman Park Sun-Kyoo.

The president's helicopter landed on an amphibious landing ship supervising the rescue. He travelled by small boat to another ship for a briefing and visited a Marine Corp base on the island to meet families of the missing.

A total of 58 crewmen were saved soon after the ship went down. But no one has been rescued since then despite a major air and sea search, which Tuesday involved 19 South Korean or US vessels, eight helicopters and 170 rescue personnel.

The incident came amid heightened tension between the two Koreas in recent weeks over a major South Korean-US military exercise in the South.

The Korea Herald said a naval attack which could spark a war would make little political sense for the North's regime, but noted the country's military had occasionally taken independent action.

"A regional commander could have sent a mini-sub or floated mines aiming at South Korean craft, with or without the backing of the high command in Pyongyang," it said in an editorial.

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