South Korea has not given up all hope of finding survivors
A mine laid by North Korea during the 1950-53 Korean War may have caused the blast which sank a warship from the South on Friday, Seoul says.
South Korean Defence Minister Kim Tae-young said the North had placed thousands of mines during the conflict - and not all of them had been cleared.
Rescuers saved 58 crew from the Cheonan and are still hunting for 46 more sailors believed to have been trapped.
Divers who reached the ship on the sea floor reported no signs of life.
Seoul has played down suggestions that Pyongyang could have deliberately sunk the ship, although Mr Kim said the possibility would be investigated.
He told a parliamentary meeting that the North had placed some 4,000 mines in its waters during the 1950-53 war.
He said most had been removed but "it must have been impossible to retrieve them all".
He added: "It is possible that a North Korean sea mine could have drifted into our area."
Another suggested cause of the sinking is that there was an accidental explosion on board.
Pyongyang has made no official comment on the incident.
'Leave no doubt'
The 1,200-tonne Cheonan naval patrol vessel sank near the disputed maritime border with the North, known as the Northern Limit Line.
Rescue officials said an explosion broke the ship into two pieces.
Some 58 crewmen were saved, and the authorities had hoped the other 46 were trapped in underwater air pockets in the wreckage.
Distraught relatives have been demanding answers from officials
But earlier on Monday, navy divers tapped on the stern of the Cheonan and heard no response.
"Divers went underwater several times, working in a very limited time of about seven to eight minutes each time, but failed to find any survivors," said a spokesman for the Joint Chiefs of Staff, Lee Ki-shik.
South Korea's official Yonhap news agency quoted officials as saying parts of the ship were able to shut out water for a maximum of 69 hours.
Two dozen military vessels, with air support, were involved in the search, said Yonhap. A US rescue vessel, with more than a dozen divers on board, has also joined the operation.
Relatives of those missing have angrily demanded progress and answers from officials.
South Korean President Lee Myung-bak earlier urged rescuers not to give up hope of finding survivors and urged officials to investigate the incident thoroughly.
South Korea recognises the Northern Limit Line, drawn unilaterally by the US-led United Nations Command to demarcate the sea border at the end of the Korean War.
But it has never been accepted by North Korea, and the area has been the scene of deadly clashes between the navies of the two Koreas in the past.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.