Image copyright Reuters Image caption Distraught relatives have repeatedly criticised the speed of the recovery operation

All 15 crew members involved in the navigation of the ill-fated South Korean passenger ferry Sewol are now reportedly in custody, facing criminal negligence charges.

The prosecution requested arrest warrants for four additional crew members. Eleven others, including the captain, had been detained earlier.

The ferry with 476 people on board sank off South Korea on 16 April.

Divers have recovered 183 bodies, but scores are missing presumed drowned.

Many of the victims were students and teachers from Danwon high school, south of Seoul.

The ferry sank on a trip from the port of Incheon to the island of Jeju.

'Incredible loss'

In the latest move on Saturday, the arrests warrants were issued for two helmsmen and two members of the steering crew.

Prosecutor Yang Jung-jin, of the joint investigation team, said the four crew members were taken into custody late on Friday, the Associated Press reported.

Image copyright Reuters Image caption Around 90 expert divers are part of the operation but only 35 of the 111 rooms have been searched so far

The 15 crew members are facing charges of criminal negligence and of failing to help passengers, the prosecution says.

On Friday, divers found 48 bodies of students wearing lifejackets in a single room on the vessel meant to accommodate just over 30 people.

The group was crammed into a dormitory and all were wearing lifejackets, a South Korean Navy officer said.

The presence of so many victims in the cabin suggested many had run into the room when the ship tilted, correspondents said.

The head of the operation to retrieve bodies said he had "no idea" how long the ship search would take.

Furious relatives have repeatedly criticised the speed of the recovery operation.

On a visit to Seoul on Friday, US President Barack Obama expressed his condolences for South Korea's "incredible loss" and offered America's solidarity.

"So many were young students with their entire lives ahead of them," Mr Obama said. "I can only imagine what the parents are going through at the moment - the incredible heartache."

The South Korean government has said it is "mobilising all available resources" towards the rescue effort.

The prosecutors are also said to be investigating whether modifications made to the ferry made it more unstable.

Factors under consideration include a turn made around the time the ship began to list, as well as wind, ocean currents and the freight it was carrying.

Reports have emerged indicating that the ship's sleeping cabins were refitted some time between 2012 and 2013, which experts say may have inadvertently affected the balance of the boat.