The captain and three other crew members of the ferry that sank off South Korea last month were indicted Thursday on charges of manslaughter through gross negligence, a prosecutor said.

Under South Korean criminal law, Captain Lee Joon-Seok, two navigators and a chief engineer could be handed the death sentence if convicted, although that penalty is very unlikely to be carried out.

"The four were indicted on charges of manslaughter through gross negligence", senior prosecutor Yang Jong-Jin, who is also spokesman for the prosecution, told AFP.

Even after being instructed by maritime safety authorities to help passengers evacuate the Sewol ferry, they failed to take any action and almost an hour later got on the first rescue boat, Yang said.

The four are accused of leaving the ship as it was sinking while telling passengers, mostly high school students on a school excursion, to stay where they were.

They scrambled to safety along with 11 other lower-ranking crew members while hundreds remained trapped, Yang said, not only failing to issue an order for passengers to leave the ship but keeping to themselves the information that a rescue boat had arrived.

They took off their uniforms and changed into civilian clothes, aware that uniformed crew members should be the last to evacuate, prosecutors were quoted as saying by Yonhap news agency.

The death penalty is rarely applied in South Korea, where a moratorium has been in place since the last execution took place in late 1997. Currently, there are some 60 people on death row.

The 11 other crew members were indicted on less serious charges including wrongfully steering the vessel, abandoning a ship and leaving passengers in a sinking boat without making efforts to rescue them, Yang said.

Ferry interior collapsing

Coastguard spokesman Ko Myung-Suk said it retrieved five bodies late Wednesday and three more Thursday. The confirmed death toll now stands at 284, with 20 still missing.

The Sewol was carrying 476 people when it sank off the southern coast on April 16 after listing sharply to one side.

Of those on board, 325 were children from a high school in Ansan City in the southern suburbs of Seoul who were on an organised trip to the southern resort island of Jeju.

Investigators addressed a number of theories Thursday on the causes behind the disaster.

The 20-year-old ferry had instability problems after it underwent renovations to add more passenger cabins two years earlier, Yang said.

Its regular captain, who was off duty on the day of the accident, had earlier told prosecutors that the ferry operator -- Chonghaejin Marine Co -- "brushed aside" repeated warnings that the 20-year-old ship had stability issues following a renovation in 2012.

On the day it sank, the ship was carrying 2,142 tonnes of cargo, despite its maximum safety load being 1,077 tonnes, Yang said, and in order to meet the obligatory waterline, 1,308 tonnes of balast water had been drained out.

When the ferry reached Maenggol Channel, notorious for fast currents, the captain was absent from the pilothouse, and a navigator and a helmsmen were in charge when it made a sharp turn, YTN TV said, citing investigators.

This caused its ill-secured cargo to shift, and the ferry to list irretrievably to one side before capsizing.

The head of the ship's operator, Chonghaejin Marine Co., and four other company officials have also been arrested.

Coastguard chief Kim Suk-Kyun said on Wednesday that divers engaged in the grisly and dangerous task of retrieving bodies were being seriously hampered as waterlogged partition walls inside the ship collapsed.

"As time goes by, the interior is caving in faster and faster, posing serious threats to divers' safety," he said.

One diver, Chun Kwang-Geun, said poor visibility inside the ship forced his colleagues to blindly grope through debris to find victims.

"If we stumble upon something, we grope it by hand (to determine whether it is a body)," said the 40-year-old who has been working on the scene since the day after the disaster.

"Many partition walls have collapsed, blocking our access," he said.

Another diver, Lee Sun-Hyong, 35, said the collapsing walls threatened to cut off air supply to divers who mostly use breathing systems tethered to the surface.