More than 280 people are missing after a ferry capsized off South Korea Wednesday, despite frantic rescue efforts involving coast guard vessels, fishing boats and helicopters.

It could be the country's biggest maritime disaster in over 20 years.

The ferry was carrying 462 people, of whom 174 have been rescued, coast guard officials said. There were four confirmed deaths and 284 people are still "unaccounted for."

There are fears that the final death toll will be high, after the 6,825-ton ship listed sharply, capsized and finally sank 20 kilometers off the southern island of Byungpoong all within two hours of sending a distress signal at 9 am local time.

"I'm afraid there's little chance for those trapped inside to still be alive," said one senior rescue team official, speaking by phone from the scene.

South Korea's Yonhap News Agency reported that after struggling for hours to get into the sunken vessel amid poor underwater visibility and strong currents, a team of Navy divers succeeded in entering the vessel and searched three of its compartments, but they found no bodies.

A second team of divers was to go underwater soon, officials said.

Additional forces and ships are on the way to the area, Yonhap quoted the Navy as saying.

The US 7th Fleet said an amphibious assault ship, the USS Bonhomme Richard which was on routine patrol west of the Korean peninsula, was being sent 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.

Several rescued passengers said they had initially been ordered to stay in their seats, before the ferry suddenly listed to one side, triggering panic.

"The crew kept telling us not to move and to stay seated," 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 passengers on board the ferry bound for the popular southern resort island of Jeju, more than 300 were students traveling with 14 teachers from a high school in Ansan just south of Seoul.

"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.

The Ministry of Security and Public Administration earlier reported that 368 people had been rescued and that about 100 were missing. But it later described those figures as a miscalculation.

There was also confusion about the total number of passengers on board, as authorities revised the figure down from 477, saying some had been double counted. It added to growing frustration and anger among families of the passengers.