Rescuers struggled with strong waves and murky waters on Thursday as they searched for hundreds of people, most of them teenagers from the same school, still missing after the South Korean ferry Sewol capsized 36 hours ago.
Coast guard, navy and private divers scoured the site of the accident, about 20 km (12 miles) off the country's southwestern coast.
Earlier, rescue teams hammered on the hull of the upturned, mostly submerged vessel, hoping for a response from anyone trapped inside, but they heard nothing, local media reported.