Rescue workers were searching by sea and air for nine people missing after a fishing boat capsized in waters off South Korea's southwestern coast, authorities said Sunday.

The 24-tonne vessel overturned in waters off the coast of Sinan county late Saturday night, according to the interior ministry.

Three crew members were rescued by a nearby boat, but nine others -- including two foreigners -- remained missing, the ministry added in a statement.

More than a dozen vessels and three aircraft from the coastguard and the navy had been deployed to search for the missing people, it said.

Most of the crew are believed to have left the capsized boat, Yonhap news agency reported, citing Interior Minister Lee Sang-min.

President Yoon Suk-yeol urged maximum efforts for the rescue operation, ordering the mobilisation of "all available resources" to widen the search area, his office said.