Grave fears are held for almost 300 passengers after a ferry carrying mostly high school students sank en route to a holiday island in South Korea.
South Korean rescue teams, as well as elite navy SEAL divers, were on Wednesday night working frantically under floodlights to find 291 people "unaccounted for" after the ferry sank with 459 people on board.
Authorities said 164 people had been rescued, with four confirmed deaths including a female crew member and a student. The death toll, however, was expected to rise.
Dramatic television footage showed terrified passengers wearing life jackets clambering into inflatable boats as water lapped over the rails of the vessel as it sank 20 kilometres off the southern island of Byungpoong.
Some could be seen sliding down the steeply inclined side of the ferry and into the water, as rescuers, including the crew of what appeared to be a small fishing boat, pulled them to safety.
The 6,825-tonne Sewol had listed violently, before it capsized and finally sank - all within two hours of sending a distress signal at 9.00am (1000 AEST).
"I'm afraid there's little chance for those trapped inside still to be alive," one senior rescue team official, Cho Yang-Bok, told YTN television as teams of divers struggled to access the submerged, multi-storey ferry.
As night fell the coastguard said the rescue operation was continuing using floodlights and underwater flares.
Several rescued passengers said they had initially been ordered to stay in their seats, but then the ferry suddenly listed to one side, triggering panic.
"The crew kept telling us not to move," one male survivor told the YTN news channel.
"Then it suddenly shifted over and people slid to one side and it became very difficult to get out," he added.
Of the 429 passengers on board the ferry bound for the popular southern resort island of Jeju, more than 300 were students travelling with 14 teachers from a high school in Ansan just south of Seoul.
Among those confirmed as rescued, 78 were students.
"I feel so pained to see students on a school trip ... face such a tragic accident. I want you to pour all your energy into this mission," President Park Geun-Hye said on a visit to the main disaster agency situation room in Seoul.
Many of the survivors were plucked from the water by fishing and other commercial vessels who were first on the scene before a flotilla of coastguard and navy ships arrived, backed by more than a dozen helicopters.
Lee said 178 divers, including a team of South Korean navy SEALS, were working at the site, but low visibility and strong currents were hampering their efforts.
The US 7th Fleet sent an amphibious assault ship on patrol in the area to help.
The cause of the accident was not immediately clear, although rescued passengers reported the ferry coming to a sudden, shuddering halt - indicating it may have run aground.
The weather was described as fine with moderate winds and sea swell.
