An overloaded ferry capsized off Indonesia's Sulawesi island, killing at least 15 people and leaving 19 others missing, rescuers said Monday.

The boat was travelling from Lanto village in Buton Central regency in Southeast Sulawesi province to nearby Lagii village when it capsized just after midnight on Monday, Buton's search and rescue agency head Muhammad Arafah said.

The wooden boat was carrying 40 people but was designed for just 20.

Rescuers were searching Monday for 19 people who were still missing in rough seas, while 15 bodies had been recovered and six people were rescued, Arafah said.

Three rubber boats, two fishing boats and six divers were deployed to search for the missing people, he said.

Thousands of residents had travelled to their villages to celebrate the regency's 9th anniversary on Sunday, and many people were transported by fishing or passenger boats.

Indonesia is an archipelago with more than 17,000 islands, and ferries are a common form of transportation. With lax safety standards and problems with overcrowding, accidents occur frequently.