The captain and crew of a South Korean ferry that capsized with hundreds of children on board acted in a way "tantamount to murder," President Park Geun-Hye said Monday, as four more crew members were arrested and the death toll rose to 87.

Park's denunciation, in which she vowed to hold all those responsible for the disaster "criminally accountable", followed the release of a transcript showing the panic and indecision that paralysed decision-making on the bridge as the ship listed and sank Wednesday morning.

The confirmed death toll jumped as divers stepped up the recovery of bodies from inside the 6,825-tonne Sewol, but 215 people remained unaccounted for.

"The actions of the captain and some crew members were utterly incomprehensible, unacceptable and tantamount to murder," Park said in a meeting with senior aides.

"Not only my heart, but the hearts of all South Koreans have been broken and filled with shock and anger," said Park, who was heckled Thursday when she met relatives of the hundreds of passengers still missing -- most of them schoolchildren.

The families have criticised the official response to the disaster, saying the initial rescue effort was inadequate and mismanaged.

The president said it was increasingly clear that Captain Lee Joon-Seok had unnecessarily delayed the evacuation of passengers as the ferry started sinking, and then "deserted them" by escaping with most of his crew members.

- Ethically 'unimaginable' -

"This is utterly unimaginable, legally and ethically," she said.

Lee was arrested on Saturday along with a helmsman and the ship's relatively inexperienced third officer, who was in charge of the bridge when the ship first ran into trouble.

Three more officers and an engineer were detained by police on Monday and prosecutors said they could face similar charges of criminal negligence and deserting passengers.

A transcript of the final radio communications between the Sewol and marine traffic control suggested a scene of total confusion as the vessel listed sharply to one side.

In the end, the evacuation order was given only around 40 minutes after the ship ran into trouble, by which time it was listing so heavily that escape was almost impossible.

"Precious minutes just wasted," was the front-page verdict of the Dong-A Ilbo daily on Monday.

Lee has insisted he had acted in the passengers' best interest, delaying the order to abandon ship because he feared people would be swept away and drowned.

Meanwhile, local TV stations aired excerpts Monday of a 2010 promotional video in which Lee said ferries offered the safest public transport "as long as you follow the instruction of our crew".

Realistic hopes of finding survivors have disappeared, but families of the missing still oppose the use of heavy cranes to lift the ship before divers have searched every section.

The United States is sending a navy salvage ship to help with the recovery of the ferry, the Pentagon said Monday, deploying the USNS Safeguard to the area from Thailand.

South Korea has not formally requested the ship, which can lift, tow and conduct diving operations, but a spokesman said it would be sent regardless as a precaution.

A US navy amphibious assault ship, the USS Bonhomme Richard, is already taking part in rescue operations.

Divers took more than two days to access the submerged ferry, and the first bodies from inside the vessel were not recovered until Sunday.

Officials said divers had rigged up multiple guide lines Monday into a dining hall as well as cabins on the third and fourth levels where many bodies were believed to be trapped.

"This allows multiple rescue workers to enter the ship at the same time to speed up operations," said Ko Myeong-Seok, a spokesman for the government task force handling the disaster.

- Survivor trauma -

Of the 476 people on board the Sewol, 352 were students from the Danwon High School in Ansan city just south of Seoul, who were on an organised trip to the holiday island of Jeju.

Cha Sang-Hoon, the head of the hospital in Ansan where 74 student survivors are being treated, said 20 percent of them were suffering from serious mental stress and depression.

South Koreans have been stunned by the tragedy, which looks set to become one of the country's worst peacetime disasters and has unleashed profound national grief.

The weeping of devastated family members could be heard across the harbour on Jindo island as boats brought the most recently recovered bodies in from the nearby rescue site Monday morning.

Wrapped in white cloth, each body was gently lifted off the boats and placed on a stretcher which was then carried away by six uniformed police wearing surgical face masks.

In a nearby tent they were placed on white tables and prepared as best as possible for the grieving relatives to make a visual identification.

The families have bitterly criticised the official response to the disaster, saying delays in accessing the submerged ship may have robbed any survivors of their last chance to make it out alive.

In her comments Monday, Park said the government should review its crisis response system.