The South Korean government will raise the wreck of the Sewol ferry, one year after it sank with the loss of more than 300 lives, most of them school children.

Bringing the Sewol has been a key demand of the victims' families, who have recently organised 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 40 metres down on the sea bed and bringing it to the surface will be technically challenging.

The ministry estimated the cost at somewhere between $US90 million ($A116.75 million) and $US140 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 on Wednesday.

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.