The 6,825-tonne ferry, Sewol, was carrying 476 passengers and crew when it sank on Wednesday while on its way to the southern popular resort island of Jeju.
Coast guard and navy divers are continuing their search after the ferry tiled over 45 degrees on the port side before it became fully capsized. Commercial ships and helicopters were also involved in the search and rescue.
So far close to 180 people have been rescued and nine confirmed dead, reports said.
The ferry had sailed out of the western port of Incheon on Tuesday evening but ran into trouble some 20 kilometres off the island of Byungpoong and the ferry had sent out a distress signal.
It remained unclear what had caused the ferry to sink and weather conditions were described as fine. Nautical charts of the wider area show reefs and shallow waters, although one government official appeared to discount the possibility the ship had hit a rock.