JAKARTA, July 24 (Reuters) - At least 15 people were killed and 19 others remained missing after a ferry sank off Indonesia's Sulawesi island on Monday, the national search and rescue agency said.

There were 40 passengers on board the ferry, which sank around midnight, the agency said in a statement, adding that the cause of the accident was still unclear.

Six passengers survived, who were being treated in a local hospital, the agency said.