At least 15 people were killed and 19 more were missing on Monday after a wooden boat sank off the coast of Indonesia's Sulawesi island, search and rescue officials said.

The boat sank with 40 people onboard just after midnight local time (1700 GMT on Sunday), the local office of Indonesia's search and rescue agency said in a statement.

Six people were rescued and taken to hospital for treatment, it said, and the cause of the sinking was being investigated.

"Provisionally, there are 19 people who are still being searched for," Muhamad Arafah, head of the local search and rescue agency in Kendari city in Southeast Sulawesi, said in the statement.

One search team will dive around the accident site, while another will search the water's surface using boats, he said.

The agency shared images of rescuers mobilising for the search effort, and several dead bodies covered by sarongs laid on tarpaulin at a local hospital.

The boat was crossing a bay between the villages of Lanto and Lagili in Central Buton regency on Muna island in Southeast Sulawesi, the local office's spokesperson Wahyudin, who like many Indonesians goes by one name, told AFP.

Wahyudin said the vessel was a wooden passenger boat and not a ferry as initially reported.

He warned the total number of passengers on board could have been higher than registered but refused to confirm local media reports the boat was overcrowded.

Indonesian media reported that villagers had travelled for a local celebration and gathered on an overcrowded boat that capsized on its way back across the bay.

Wahyudin said the agency would provide an update on the cause and missing passengers later on Monday.

It is common in Indonesia for the number of actual passengers on a boat to differ from the manifest.

Marine accidents occur frequently in the Southeast Asian archipelago nation of around 17,000 islands, where people rely on ferries and small boats to travel around despite poor safety standards.

In 2018, more than 150 people drowned when a ferry sank in one of the world's deepest lakes on Sumatra island.

In May last year, a ferry carrying more than 800 people ran aground in shallow waters off East Nusa Tenggara province and remained stuck for two days before being dislodged.

No one was hurt in that accident.