South Korean PM resigns over sunken ferry

South Korea's prime minister has resigned over the sinking of a passenger ferry that left more than 300 people dead or missing, amid huge public anger over what is seen as a bungled response to the tragedy.

Chung Hong-Won admitted he had not been up to the task of overseeing rescue operations after the Sewol capsized with 476 people - many of them schoolchildren - on board.

"I offer my apology for having been unable to prevent this accident from happening and unable to properly respond to it afterwards," he said.

"I believed I, as the prime minister, certainly had to take responsibility and resign."

Parents and relatives of the missing and the dead have blasted the response to the sinking, saying the rescue operation was too slow to swing into action, possibly costing lives.

There has also been rage over perceived corruption and lax safety standards that may have led to the disaster, with claims that the ferry was overloaded and the passenger list was inaccurate and incomplete.

"The latest accident left all South Koreans in great shock and sorrow. (Many) days have passed since the accident, but the screams of families of the missing still keep me up at night," Chung told a nationally-televised press conference.

"I sincerely hope that South Koreans and families of the Sewol victims will forgive and understand me for being unable to fulfil my obligations until the end. I beg you once again to rally support (for rescue operations)."

The confirmed death toll from the tragedy stood Sunday at 187, with 115 unaccounted for, their bodies believed trapped in the sunken vessel.

Divers were battling decompression sickness and atrocious weather in their grim search for corpses.

Further complicating their efforts was the increasing depth of the wreck as it sinks slowly into the silt of the seabed, making an already dangerous diving operation even more hazardous.

Despite waves up to three metres and near gale-force winds, teams were still trying to search the ferry.

A coastguard spokesman said 98 frogmen would be trying to get into rooms on the fourth deck of the 6,825 tonne Sewol, but he warned the worsening conditions were making things hard.

"A growing number of divers are reporting decompression sickness," he said. "We are trying our best to make progress in these difficult weather conditions."

"As the ship has sunk further... the diving depth also increased to more than 40 metres, posing even more difficulties for search efforts."

On the surface, recriminations continue, with four more of the ship's crew arrested on Saturday.

Their arrests mean all 15 surviving crew members responsible for sailing the vessel were in custody, facing charges ranging from criminal negligence to abandoning passengers.

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