JINDO (AFP) - South Korean rescuers and dive teams worked frantically under floodlights as fears rose for nearly 300 people missing after a ferry sank on Wednesday with 475 on board, mostly high school students bound for a holiday island.

Yonhap news agency said 179 people had been rescued, citing national coastguard officials, leaving 290 unaccounted for. There were six confirmed deaths, including a female crew member and a student.

There are concerns the death toll could rise sharply. The 6,825-tonne Sewol listed violently, capsized and finally sank - all within two hours of sending a distress signal at 9.00am (0000GMT).

"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 divers struggled to access the submerged multi-deck ferry.

Dramatic television footage showed terrified passengers wearing life jackets clambering into inflatable boats with water lapping over the rails of the vessel as it sank 20km off the southern island of Byungpoong.

Some slid 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.

As night fell, the coastguard said the rescue operation was continuing using floodlights and underwater flares.

"We won't give up, although the situation is extremely worrying," a coastguard spokesman said.

Several rescued passengers said they had initially been told to remain in their cabins and seats, but then the ferry listed hard 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.

The passengers included 375 students from a high school in Ansan just south of Seoul, who were travelling with 14 of their teachers to the popular island resort of Jeju, Yonhap said.

"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 national disaster agency's 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.

Mr Lee Gyeong Og, the vice-minister of security and public administration, said 178 divers, including a team of South Korean navy Seals, were working at the site, but low water visibility and strong currents were hampering their efforts.

The US 7th Fleet sent an amphibious assault ship on patrol in the area to help while White House spokesman Jay Carney said Washington was ready to provide its ally with "any assistance" needed.

One local official, who had taken a boat to the site an hour after the distress signal was sent, said he was "very concerned" about those still missing.

"The ship was already almost totally submerged when I got there. A lot of people must have been trapped," the official, who declined to be identified, told Agence France-Presse by phone.

The cause of the accident in fine weather was not immediately clear, although rescued passengers reported the ferry coming to a sudden, shuddering halt - indicating it may have run aground.

"I heard a big thumping sound and the boat suddenly started to tilt," one rescued student said.

Another spoke of luggage and vending machines crashing down on passengers as the vessel tipped over.

"Everyone was screaming and a lot of people were bleeding badly," he said.

Distraught parents gathered at the high school in Ansan, desperate for news, with some yelling at school officials while others repeatedly tried to call their children's mobiles.

"I'm so worried about my son," said one father, Mr Lee Ki Hong. "I texted him an hour before the ship sank, but there has been no reply," he told YTN.

Survivors were taken to a gymnasium on nearby Jindo island, where relatives of the missing, wrapped in blankets against the cold, were holding what looked set to be a night-long vigil on the quay of the main harbour.

Three giant floating cranes had been despatched to the site and would begin operations to raise the submerged vessel tomorrow, officials said.

Scores of ferries ply the waters between the South Korean mainland and its multiple offshore islands every day, and accidents are relatively rare.

In one of the worst incidents, nearly 300 people died when a ferry capsized off the west coast in October 1993.