South Korean President Park Geun-hye (C) prays during in memory of victims of the sunken Sewol ferry at a church in Seoul, May 18, 2014. [Photo/Agencies]

SEOUL - South Korean President Park Geun-hye formally apologised on Monday for a ferry disaster last month that killed about 300 passengers, mostly school children, and said she would break up the coast guard because it had failed in its rescue mission.

The coast guard's rescue duties will be transferred to a national emergency safety agency to be set up and the national police will take over its investigative function, Park said in a televised national address.

"I apologise to the nation for the pain and suffering that everyone felt, as the president who should have been responsible for the safety and lives of the people," Park said.

The Sewol, with 476 passengers and crew members on board, capsized and sank on April, killing at least 286 people. Eighteen people remain missing after South Korea's worst civilian maritime disaster in 20 years.

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Of the passengers, 339 were children and their teachers on a field trip from a high school on the outskirts of Seoul. Only 172 people were rescued, with the rest presumed to have drowned.

Some of the crew, including the captain, were caught on videotape abandoning ship while the children were repeatedly told to stay put in their cabins and await further orders.

Park has apologised in person to many family members of the victims but her administration has faced continued criticism and nationwide anger for its handling of the disaster, when a faster initial response could have saved more lives.

She said the coast guard had not only failed in its search and rescue duty but that, in its current form, it would be unable to prevent another large-scale disaster.

"The coast guard continued to get bigger in size but did not have enough personnel and budget allocated for maritime safety, and training for rescue was very much insufficient," Park said.

All 15 surviving crew members were indicted last week, including the captain and three senior crew members on homicide charges. The remaining 11 crew were also indicted for negligence.

The prosecution says the ferry was structurally defective after a remodelling to add capacity and was massively overloaded with cargo. A sharp turn then caused it to list and capsize, they said.

The Sewol had been on a supposedly routine journey from the mainland port of Incheon south to the holiday island of Jeju.