The overloaded passenger boat was carrying more than 60 people when it overturned Monday in rough seas near Madura island, off Java’s north coast.

Indonesia’s search and rescue agency said 18 people had died, including several children, with 39 survivors.

A fleet of rescue vessels, a plane and local fishing boats are searching the waters for at least four others still missing, authorities said.

But local reports have suggested that there could be more victims who were not listed on the official manifest.

“It’s suspected that the boat was overloaded,” East Java Police spokesman Frans Barung Mangera said.

“The boat was hit by a large wave, overturned and sank.”

The 10-metre long traditional vessel was found by fishermen shortly after it was swamped, he added.

Marine accidents are common in Indonesia, an archipelago nation of more than 17,000 islands, where many use ferries and other boats to travel despite poor safety standards.

More than 160 people died when a passenger ferry sunk into the depths of one of the world’s deepest lakes on Sumatra island last year.

In 2009, over 300 people are estimated to have drowned when a ferry sank between the islands of Sulawesi and Borneo.