At least 15 people are dead after a ferry sank off Indonesia’s Sulawesi island, according to local authorities.

The ferry had 40 passengers on board when it sank at approximately midnight (16:00 GMT) and authorities are searching for 19 people who remain missing, Indonesia’s national search and rescue agency said on Monday.

Six passengers were rescued and are being treated in hospital, the agency said.

It was not clear how many people were on board as it is not uncommon in Indonesia for the actual number of passengers on a boat to differ from the manifest.

“The search will be conducted by dividing into two teams. The first team will dive around the accident site,” Muhamad Arafah, head of the local search and rescue agency in Kendari city in Southeast Sulawesi, said in a statement.

“The second team will conduct a sweep above the water surface around the accident site using a rubberboat and longboat.”

Ferry accidents are not uncommon in Indonesia, which is the world’s largest archipelago country with more than 17,000 islands.

In 2018, as many as 192 people drowned when an overloaded ferry overturned and sank on Lake Toba on Sumatra island.

In May last year, a ferry with more than 800 people on board ran aground in waters off East Nusa Tenggara province. No one was hurt in the incident. (Al Jazeera)

•PHOTO:The sinking of a ferry on Lake Toba on Sumatra island in 2018 killed more than 150 people [File: Antara Foto/Irsan Mulyadi/via Reuters]