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

Some 183 bodies have been recovered from the Sewol, but scores of people are missing, presumed drowned.

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

There were 476 people on board, with many trapped inside as the ferry listed and sank within two hours of distress signals being sent. A total of 174 passengers were rescued.

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

Many of those who died or are presumed dead were students and teachers from Danwon high school, south of Seoul.

Furious relatives attacked the speed of the recovery operation on Friday in a confrontation with the fisheries minister and the coastguard chief.

In a briefing to reporters on the southern island of Jindo, Navy Captain Kim Jin-Hwang described the difficult conditions that the divers were facing.

He said one group had found the single dormitory room filled with the bodies of 48 students wearing lifejackets. The presence of so many victims in the cabin suggested many had run into the room when the ship tilted, correspondents said.

"It's very stressful," Kim said, adding that the divers were all too aware of criticism over the speed of the search.

President Obama proposed holding a moment of silence to honour the victims of the ferry disaster

Retrieving the bodies was far harder than finding them, he said, with divers unable to spend much longer than 10 minutes inside the ship at a time.

"Just imagine a room that is flipped," one of the divers told the Associated Press news agency. "Everything is floating around, and it's hard to know exactly where they are."

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."

Officials said rescuers are retrieving around 30 bodies a day but the bereaved families have demanded that all remaining bodies are removed from the ferry before the weekend.

Relatives of passengers who died or remain missing are furious at the slow speed of the search operation

Search officials said just 35 of the 111 rooms had been searched so far.

The government says it is "mobilising all available resources" towards the rescue effort but bad weather and stronger currents due on Saturday and Sunday are expected to hamper their efforts.

Prosecutors are 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.

Investigators on Friday said that life rafts and escape chutes on a sister ship to a sunken ferry were not working properly.

The ferry's captain and 10 crew members have been arrested on charges ranging from criminal negligence to abandoning passengers.

Prosecutors have also raided several businesses affiliated with the ferry operator, the Chonghaejin Marine Company, as part of an overall probe into corrupt management.