The confirmed death toll in South Korea's ferry disaster has risen above 100.

The official toll provided by the coastguard stands at 120, with 182 still missing nearly a week into the rescue and recovery effort.

The Sewol vessel was carrying 476 people, most of them schoolchildren, when it capsized and sank last Wednesday.

A total of 174 people, including the captain and most of his crew, were rescued.

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

Crew members detained over the ferry disaster said they had done their best to launch life rafts, and one suggested possible technical reasons for the ship capsizing.

The ferry had 29 crew, including its captain Lee Jeon-Sook, and 20 of them escaped the ferry as it sank.

There has been public outrage at reports they were among the first to evacuate while hundreds remained trapped in the vessel.

Captain Lee and two crew members were arrested over the weekend and charged with criminal negligence, while another four crew members were taken into police custody yesterday.

Those four were paraded, heads bowed and hiding their faces, before television cameras today.

Asked why only one of the Sewol's 46 life rafts had been used, one said conditions had made their deployment impossible.

"We tried to gain access to the rafts but the whole ship was already tilted too much," he said.

"We tried to launch the life rafts but it was hard to get to where they were," another said.

One crew member, apparently an officer, suggested the ferry had a structural flaw that made it difficult to regain equilibrium once it had been lost.

The ship was built in 1994 in Japan and purchased by the Cheonghaejin Marine Company in 2012.

The officer also mentioned "some errors" with the steering system.

The Sewol capsized after making a sharp right turn.

This led experts to suggest its cargo manifest might have shifted, causing it to list beyond a critical point of return.

The expected final death toll of nearly 300 would make this one of South Korea's worst peacetime disasters.