"Our navy divers knocked on the stern with hammers in the afternoon, but so far there is no response from the inside," said defence ministry spokesman Won Tae-Jae.

A mystery explosion tore the 1,200-tonne corvette in half in the Yellow Sea Friday night near the tense disputed border with North Korea, in one of the country's worst sea disasters.

Seoul officials say there is no evidence so far Pyongyang attacked the Cheonan.

However, Defence Minister Kim Tae-Young said a drifting North Korean mine dating back to the 1950-53 war might have caused the blast, or the North might have intentionally sent a mine floating towards the ship.

High waves, poor visibility and strong currents thwarted efforts over the weekend to dive on the separate sections of the 88-metre (290 foot) ship.

Rescuers said most of the missing crew members would have been in the stern section. There had been speculation some may have survived in air pockets in watertight compartments.

A total of 58 crewmen were saved soon after the ship went down in near-freezing waters off Baengnyeong island near the disputed border, scene of deadly naval clashes in 1999 and 2002 and of a firefight last November.

No one has been rescued since then despite a major air and sea search.

Fourteen navy craft and six coastguard ships backed up by aircraft were involved Monday, plus a 3,200-tonne US salvage ship with 15 divers.

In Washington, a Pentagon spokesman said the US Navy would be providing further assistance.

"At the request of the Republic of Korea, US Navy forces are going to be supporting the republic of Korea navy search, recovery and salvage efforts. It includes a number of vessels," said Pentagon spokesman Bryan Whitman.

During a briefing at a navy command in Pyeongtaek, south of Seoul, anguished family members, some weeping, demanded a faster rescue operation.

"We are running out of time," said a despairing woman in her 40s.

Another woman collapsed during the briefing and was taken to hospital. There was a brief scuffle when soldiers stopped angry relatives from breaking into an office.

President Lee Myung-Bak has called four emergency security meetings since the sinking but cautioned against jumping to conclusions about the cause.

"Do not give up hope that there could be survivors," he told searchers earlier Monday in a statement. "Look into the causes of the incident thoroughly and leave no single piece of doubt behind."

Theories about the cause range from an explosion inside the corvette, which was reportedly carrying torpedoes, depth charges, missiles and other weaponry; a drifting mine possibly dating back to the war; or a torpedo attack from the North.

US and South Korean military officials say no unusual movements have been detected by the North, which has made no mention of the incident.

Defence Minister Kim told parliament's defence committee there were no signs of a torpedo attack before the explosion, citing accounts of rescued sailors who were operating the ship's radar.

"It is possible that a North Korean sea mine could have drifted into our area," he said.

The North brought in about 4,000 mines from the Soviet Union during the war and placed about 3,000 of them in the Yellow Sea and the Sea of Japan (East Sea), he said.

"Though many mines were removed, it must have been impossible to retrieve them all," Kim said.

Kim said investigators could not rule out the possibility of a mine which came adrift from its moorings. "Or we have to see whether North Korea has intentionally set a mine adrift," he added.

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