South Korean President Park Geun-hye (2nd R) looks at portraits of missing passengers onboard sunken ferry Sewol during her visit to a port in Jindo, on the occasion of the first anniversary of the ferry disaster that killed more than 300 passengers, April 16, 2015.[Photo/Agencies]

SEOUL - South Korea said on Wednesday it will raise the Sewol ferry that sank a year ago, killing more than 300 people, most of them children, yielding to pressure from mourning families who have called for a deeper investigation into the disaster.

The Sewol, which was structurally unsound, overloaded and travelling too fast on a turn, capsized and sank during a routine voyage and lies 44 metres (144 feet) deep 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 seen on TV escaping the sinking vessel.

A government committee concluded that it would be possible to raise the 6,800-tonne vessel at a cost of 150 billion won ($139 million), the government said in a statement.

The work, which the government hopes can begin in September, could take up to 18 months and the cost could rise beyond 200 billion won depending on weather conditions and technical difficulties, it said.

"The primary risk is that the Sewol is a vessel built more than 20 years ago so there is corrosion in its body," Park In-yong, the retired navy admiral who heads the newly formed Ministry for Public Safety and Security, told a briefing.

"And it is lying on its left, so as we try to raise it without righting it, there may be structural weakening."

Raising the Sewol has been a central demand of victims' families, some of whom say the government let them down by failing to announce a salvage plan by the first anniversary of the disaster on April 16. Nine of the victims' bodies remain missing.

The decision comes as President Park Geun-hye is under renewed political pressure, with her prime minister offering his resignation this week after a businessman and former lawmaker who committed suicide accused him of accepting illegal funds.

Prime Minister Lee Wan-koo has denied he received the money from businessman Sung Wan-jong, but Park is widely expected to accept his resignation.

On Saturday, thousands of people including family members held a rally in central Seoul to protest what they said was government incompetence and foot dragging over raising the ship and allowing an independent probe into the disaster.

Park's government was criticised for its response to the sinking, including a rescue effort widely seen as botched. ($1 = 1,080 won)