South Korean President Park Geun-hye has said the captain and some crew members of the sunken ferry committed "unforgivable, murderous acts" in the disaster, which left more than 300 people dead or missing.

The captain initially told passengers to stay in their rooms and waited more than half an hour to issue an evacuation order as the ferry Sewol sank.

By then the ship had tilted so much that many of the roughly 240 people missing are believed to be trapped inside.

Ms Park said the captain and crew "told the passengers to stay put but they themselves became the first to escape, after deserting the passengers".

She said that "legally and ethically, this is an unimaginable act.

Earlier, four more crew members were detained on allegations of failing to protect passengers.

Senior prosecutor Ahn Sang-don said two first mates, one second mate and a chief engineer are also accused of abandoning the ship.

Prosecutors are considering whether to ask a court for a formal arrest warrant that would allow for a longer period of investigation. South Koreans can only be detained for 48 hours without a court-issued formal arrest warrant.

The ferry's captain and two other crew members were previously formally arrested on suspicion of negligence and abandoning people in need.

Sixty-four bodies have been recovered from the vessel.

A radio transcript revealed the ferry was crippled by confusion and indecision well after it began listing.

About 30 minutes after the Sewol began tilting, a crew member asked a marine traffic controller whether passengers would be rescued if they abandoned ship off South Korea's southern coast. The crew member posed the question three times in succession.

That followed several statements from the ship that people aboard could not move and another in which someone declared that it was "impossible to broadcast" instructions.

Many people followed the captain's initial order to stay below deck, where it is feared they remain trapped.

"Even if it's impossible to broadcast, please go out and let the passengers wear life jackets and put on more clothing," an unidentified official at Jindo Vessel Traffic Services Centre urged at 9.24am on Wednesday, 29 minutes after the ferry first reported trouble, according to the transcript released by South Korea's coast guard.

"If this ferry evacuates passengers, will you be able to rescue them?" the unidentified crew member asked.

"At least make them wear life rings and make them escape." the traffic-centre official responded.

"If this ferry evacuates passengers, will they be rescued right away?" the crew member asked again.

"Don't let them go bare - at least make them wear life rings and make them escape." the traffic official repeated. "The rescue of human lives from the Sewol ferry ... the captain should make his own decision and evacuate them. We don't know the situation very well. The captain should make the final decision and decide whether you're going to evacuate passengers or not."

"I'm not talking about that," the crew member said. "I asked, if they evacuate now, can they be rescued right away?"

The traffic official then said patrol boats would arrive in 10 minutes, though another civilian ship was already nearby and had told controllers that it would rescue anyone who went overboard.

The ferry sank with 476 people on board, many of them students from a single high school.

The cause of the disaster is not yet known, but prosecutors have said the ship made a sharp turn before it began to list.

More than 170 people survived the sinking of the Sewol, which had been on its way from the South Korean port city of Incheon to the southern island of Jeju.

The captain took more than half an hour to issue an evacuation order, which several passengers have said they never heard.

The confirmed death toll jumped over the weekend after divers finally found a way inside the sunken vessel and quickly discovered more than a dozen bodies. They had been hampered for days by strong currents, bad weather and low visibility.

The South Korean news agency Yonhap reported that another body was recovered early on Monday near the sunken ship.

Families of the missing are staying on Jindo Island, where information sheets taped to the walls of a gymnasium offered details to help identify any corpses, including gender, height, length of hair and clothing.

It was too little for Lee Joung-hwa, a friend of a crew member who is among the missing.

"If only they could have made some kind of image of the person's face. Who can tell who this person is just by height and weight?" Lee said.

A woman with a blue baseball cap shouted at government officials who were seated nearby, working at their desks. "I can't live like this. I'm so anxious." she yelled. "How can I trust the police?"

Anguished families, fearful they might be left without even their loved ones' bodies, vented rage on Sunday over the government's handling of the crisis.

About 100 relatives attempted a long protest march to the presidential Blue House in Seoul, about 250 miles to the north, saying they wanted to voice their complaints to President Park Geun-hye. They walked for about six hours before police officers in neon jackets blocked a main road.

"The government is the killer," they shouted as they pushed against a police barricade.

"We want an answer from the person in charge about why orders are not going through and nothing is being done," said Lee Woon-geun, father of 17-year-old missing passenger Lee Jung-in. "They are clearly lying and kicking the responsibility to others."

Earlier on Sunday, relatives of the missing blocked the car of Prime Minister Chung Hong-won and demanded a meeting with Park as Chung made a visit to Jindo. Chung later returned to the gymnasium, but met only with a number of representatives of the family members in a side office.

On Sunday evening, dozens of relatives who gathered at the port in Jindo surrounded the fisheries minister, Lee Ju-young. They shouted, swore, yelled threats and pushed him as he was on his way to a meeting with other officials.

Relatives are desperate to retrieve bodies before they decompose beyond recognition, Lee Woon-geun said.

"After four or five days, the body starts to decay. When it's decayed, if you try to hold a hand, it might fall off," he said. "I miss my son. I'm really afraid I might not get to find his body."

The Sewol's captain, Lee Joon-seok, 68, was arrested on Saturday, along with one of the ship's three helmsmen and the 25-year-old third mate. The third mate was steering at the time of the accident, in a challenging area where she had not steered before, and the captain said he was not on the bridge at the time.

Senior prosecutor Yang Jung-jin said the third mate has refused to tell investigators why she made the sharp turn. Prosecutors have not named the third mate, but a fellow crew member identified her as Park Han-kyul.

As he was taken from court in Mokpo on Saturday, the captain explained his decision to wait before ordering an evacuation.

"At the time, the current was very strong, the temperature of the ocean water was cold," Lee told reporters, describing his fear that passengers, even if they were wearing life jackets, could drift away "and face many other difficulties".

He said rescue boats had not yet arrived, and there were no civilian vessels nearby.