In a last terrified, tearful call to his elder sister, 16-year-old Kim Dong-Hyup promised to get off the sinking ferry alive.

Four days later, his family still can't accept the missing high school student may never come home.

Even as divers began retrieving bodies from the submerged ferry that capsized and sank on Wednesday morning, Kim's father refused to give up hope.

"He's always been such a strong-willed boy. I really believe he'll make it somehow," Kim Chang-Gu, 46, told AFP.

"When he comes back home, I'll buy him a lot of chicken. Chicken is his favourite. When he left for the trip, I bought him two chickens but he couldn't finish them off," Kim said.

The teenager was one of 352 students from Danwon High School in Ansan city, just south of Seoul, who were on the ferry bound for the popular holiday island of Jeju.

The precise cause of the disaster is still under investigation, but shortly after the ferry sent out a distress call it began to list sharply to one side.

Within 30 minutes it was at a 90 degree angle and then inverted completely before sinking.

The first the family knew of the unfolding tragedy was a call Kim Dong-Hyup made to his elder sister, Kim Ha-Na.

"He said the situation was getting more and more serious, with the ship lying on its side," his 22-year-old sister recalled.

'He was being very brave'

"He said they'd been told not to move and were waiting with their lifejackets on," Kim said.

"And then he told me: 'I don't really know what's going on, but what I can say is I'm a man and I'll make it," she added tearfully.

"He was being very brave, but I could tell he was scared and I could hear other children in the background, weeping and crying out."

The captain of the ferry, who was arrested on Saturday, has been criticised for delaying the evacuation of the ferry.

The captain said he had initially ordered passengers to stay where they were because the sea conditions were bad and he felt they would be "swept away" before rescue boats arrived.

When the order finally came to abandon ship, many were unable to climb out of the ship because it was listing over so sharply.

Kim Chang-Gu said his son had dreamed of becoming a movie actor and that his main interest in school was the drama club of which he was an avid member.

"We aren't well off, and he said that when he became a film star, he'd make me rich and comfortable," Kim said.

"When he comes back, I'll do everything I can to get him acting lessons," he added.

The confirmed death toll from the disaster stood at 49 on Sunday morning, with 253 people still unaccounted for.

Relatives of the missing, like Kim's family, have been sleeping in a gymnasium on Jindo island -- not far from the disaster site -- since the ferry sank.

Many had clutched at the slim home that some of the passengers might have been able to stay alive in air pockets in the upturned vessel.

But after four days, few shared Kim's conviction that their loved ones may still be alive.

The Kim family made a boat trip to the site of the submerged ferry, feeling the need to get as close as possible even if there was nothing comforting to see.

"When I was there, I heard his voice," Kim Chang-Gu said. "He was calling, 'father, father',".