Divers are continuing to search the wreckage of the ship
South Korea's defence minister has renewed speculation a torpedo could have caused the blast that sank a warship off its coast last week.
Kim Tae-young said it was a "likely possibility" the Cheonan had been hit by such a missile, but that all possibilities needed to be considered.
Mr Kim did not say who would have fired a torpedo or under what circumstances it could have happened.
Rescuers are still searching for 46 sailors missing on board the ship.
Officials say some could be trapped alive in watertight sections of the vessel.
"We'd like to think our sailors were well-trained enough to survive days underwater," said Cdr Song Moo-jin of South Korea's naval salvage unit.
Mr Kim had previously said a sea mine could have caused the blast, which tore the 1,200-tonne corvette in half near Baengnyeong Island, close to the maritime border with North Korea.
But on Friday, he told parliament a torpedo was "a more realistic cause than a mine".
Mr Kim said sailors who had escaped the ship had not reported detecting any incoming missiles before the blast, the Yonhap news agency reported.
Earlier in the week, South Korea's President Lee Myung-bak said there had to be "no suspicion or negligence" in the investigation.
Shortly after the sinking, Seoul said it did not believe Pyongyang had fired on the ship.
There were initial reports another South Korean ship had fired shots toward an unidentified vessel after the blast, but officials later speculated the target had been a flock of birds.
Officials have said establishing a definitive cause could have to wait until the ship is salvaged.
The search for the missing sailors continued on Friday, after poor weather disrupted operations on previous days.
Yonhap said 169 military divers were searching the area throughout the day although conditions were still difficult.
Oxygen has been piped into the vessel, but divers have detected no signs of life.
Pyongyang has made no official comment on the incident.
It does not accept the maritime border, known as the Northern Limit Line, which was drawn unilaterally by the US-led United Nations Command at the end of the Korean War.
The sea border has been the scene of deadly clashes between the navies of the two Koreas in the past.