More than a dozen of vessels are involved in the rescue operation
South Korean search teams have suspended their efforts to reach missing sailors on a sunken warship because of poor weather conditions.
The move comes a day after a diver died searching the wreckage of the Cheonan, which is lying near Baengnyeong Island, close to the border with North Korea.
Forty-six sailors have been missing since an explosion split the ship in two late on Friday.
A minister said the blast could have been caused by a North Korean mine.
A defence ministry spokesman in Seoul told the Yonhap news agency that waves at the rescue site were up to 2m (6ft) high, with winds blowing at 10 knots.
"We are temporarily suspending operations. We cannot expect to get near the ship in this condition," said Won Tae-Jae.
The diver who died, one of dozens trying to gain access to the wreckage, reportedly lost consciousness under water. Yonhap said two of the others were in hospital.
On Tuesday, a navy spokesman said the divers were working in "a very vicious environment" with swift currents and poor visibility.
"Our goal is to get into the ship and find any survivors, but at the moment it is extremely hard to do so," he said.
Fifty-eight crew members were rescued as the 1,200-tonne corvette Cheonan sank.
Officials say others could have survived in water-tight cabins in its stern, and oxygen has been piped through cracks in the vessel to increase their chances of survival.
But divers who reached the ship on Monday reported hearing no response when they tapped on the hull.
More than a dozen South Korean ships and a US vessel are involved in the rescue effort.
South Korean President Lee Myung-Bak visited the scene of the wreck on Tuesday and ordered the military onto alert, saying: "Since the sinking took place at the front line, the military should thoroughly prepare for any move by North Korea."
Government officials have also been told not to take leave until tensions over the sunken warship ease.
Defence Minister Kim Tae-Young has said that the explosion could have been caused by a mine laid by the North during the 1950-53 Korean War - or intentionally sent floating towards the South Korean vessel by the communist state.
The Navy's chief of staff, Kim Sung-Chan, has said the ship's ammunition storage room did not appear to have exploded and "the ship was broken in two because of powerful outside pressure or an (exterior) explosion", AFP news agency reported.
But military officials said establishing a definitive cause might have to wait until the ship is salvaged.
The defence minister said this could happen as early as next week, along with the establishment of a fact-finding commission.
"We will explain anything to answer questions and address rumours concerning the incident," he said on Wednesday. "We have nothing to hide and no reason to hide. So many lives are involved in this case."
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 and has been the scene of deadly clashes between the navies of the two Koreas.