SEOUL (AFP) - The South Korean government announced on Wednesday it would raise the wreck of the Sewol ferry, one year after it sank with the loss of more than 300 lives, most of them schoolchildren.

Bringing the Sewol to the surface has been a key demand of the victims' families, who have stepped up their campaign in recent weeks, organising a series of large-scale protests that have seen clashes with riot police using water cannon and pepper spray.

The Ministry of Public Safety and Security said the salvage operation would likely begin in September, off the southwestern island of Jindo, where the Sewol sank on April 16 last year.

The 6,825-tonne vessel lies 40m down on the sea bed and bringing it to the surface represents a substantial technical challenge.

The ministry estimated the cost at somewhere between US$90 million (S$120 million) and US$140 million and said the operation could take as long as one-and-a-half years.

"We will take thorough measures not to lose the bodies of the missing," Maritime Minister Yoo Ki-June told a press briefing.

The overloaded Sewol was carrying 476 people, including 325 students from the high school in Ansan, when it sank. Only 75 students survived.

A total of 295 bodies were recovered from the ferry, but nine remained unaccounted for when divers finally called off the dangerous search in November.

The families of those still missing had spearheaded the calls for the ferry to be brought to the surface.

Yoo said the maritime ministry would immediately begin a bidding process to select a company to handle the salvage operation.

"Once the company is selected, a detailed salvage plan... will be drawn up over the next three months," the minister said.

South Korean President Park Geun-Hye, whose popularity ratings plunged after her administration was widely criticised for its response to the disaster, promised on the first anniversary of the tragedy last week that all efforts would be made to salvage the ship.

Although the decision meets one of their key requests, the victims' families have vowed to continue their protests in the coming weeks to push their demand for a fully independent inquiry into the sinking.

While largely blamed on the ship's illegal redesign and overloading, the accident laid bare deeper-rooted problems of corruption, lax safety standards and regulatory failings attributed to the country's relentless push for economic growth.