More than 280 people, many of them students from the same high school, were missing after a ferry capsized off South Korea yesterday, in what could be the country’s biggest maritime disaster in over 20 years.

The ferry was carrying 462 people, of whom 174 have been rescued, coastguard officials said. Four people were confirmed dead, but as frantic rescue operations continued late into the night under light from flares, hopes were fading for the 284 unaccounted for.

It was not immediately clear why the Sewol ferry listed heavily on to its side and capsized in apparently calm conditions off South Korea’s southwest coast, but some survivors spoke of a loud noise prior to the disaster.

“It was fine. Then the ship went ‘boom’ and there was a noise of cargo falling,” said Cha Eun-ok, who was on the deck of the ferry taking photographs at the time.

“The on-board announcement told people to stay put ... people who stayed are trapped,” she said in Jindo, the nearest town to the scene of the accident.

The families of those still missing faced agonising uncertainty as divers searching for those trapped in the largely submerged ship were forced to suspend their work until daybreak today.

Survivors in Jindo huddled on the floor of a gymnasium, wrapped in blankets and receiving medical aid. One woman lay on a bed shaking uncontrollably. A man across the room wailed loudly as he spoke on his mobile phone. Furious relatives of the missing threw water at journalists trying to speak to survivors and at a local politician who had arrived at the makeshift clinic.

Most of the passengers on board the ferry appeared to have been teenagers and their teachers from a high school near Seoul who were on a field trip to Jeju island, about 100 kilometres south of the Korean peninsula. An official from the Danwon High School in Ansan, a Seoul suburb, said earlier that all of its 338 students and teachers had been rescued.

The on-board announcement told people to stay put but the people who stayed are trapped

But that could not be confirmed by the coastguard or other officials involved in the rescue, and did not appear to tally with more up-to-date assessments of survivor numbers. The school official asked not to be identified.

Adding to the confusion, the Ministry of Security and Public Administration initially reported that 368 people had been rescued and that about 100 were missing.

But it later described those figures as a miscalculation, turning what had at first appeared to be a largely successful rescue operation into potentially a major disaster.

There was also uncertainty 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.

Desperate parents gathered outside Danwon school when news of the disaster first broke in the morning, and fought their way on to coaches provided to take them on to Jindo.

Park Seong-ho, the father of a 17-year-old boy who had been on board the ferry but who had not been in contact, said before leaving: “I have to go now. It is as if the world is falling apart. I really want to go now to see my son.”

Jeong Kyung-mi, mother of another 17-year-old from the school, was more fortunate. She received a text message from her son saying he had been rescued with friends and was safe.

“When I heard the news, it felt like my heart had stopped beating.”

The ferry began to list badly about 20 kilometres off the southwest coast as it headed for Jeju.

A member of the crew of a local government ship involved in the rescue, who said he had spoken to members of the sunken ferry’s crew, said the area was free of reefs or rocks and the cause was likely to be some sort of malfunction on the vessel.

There were reports of the ferry having veered off its course, but coordinates of the site of the accident provided by port authorities indicated it was not far off the regular shipping lane.