SEOUL - South Korea prepared on Sunday to raise the Sewol ferry that sank nearly three years ago, killing more than 300 people, most of them children, testing a system to bring the ship to the surface in the hope of finding the last nine bodies.

The Sewol, which was structurally unsound, overloaded and traveling too fast on a turn, capsized and sank during a routine voyage on April 16, 2014. It lies at a depth of 44 metres, off the southwestern island of Jindo.

Of those killed, 250 were teenagers on a school trip, many of whom obeyed crew instructions to remain in their cabins even as crew members were escaping the sinking vessel.

Mourning families have been calling for the ship to be raised and for a more thorough investigation into the disaster.

"The main reason is to find the nine missing bodies," a Ministry of Oceans and Fisheries official, who declined to be identified, told Reuters, referring to the salvage that is costing about 85 billion won ($75 million).

A Chinese company has fitted 33 beams beneath the hull, which will be raised by 66 hydraulic jacks, he and a second ministry official said.

The system was tested on Sunday and it was hoped an attempt could be made to lift the ship the next time the tides were right, on around April 4-5, the officials said.

"We're waiting for the neap tide and when it comes, we'll see what the weather is like and we'll try," the first official said.

"We're not making any promises but we'll try," he said. "There have to be low waves for at least three days."

The disaster was a blot on former president Park Geun-hye's record in office that she was never able to clear. She was accused of dithering after the initial report came in that the ferry was in trouble, accusations she denied.

Her response to the disaster was again raised in recent months after she came under suspicion in the course of an investigation into a corruption scandal that led to her dismissal from office on March 10.