A boat carrying 93 Indonesian migrant workers from Malaysia has sunk off Batam Island just south of Singapore, killing 17 of them according to Indonesia’s national disaster agency.

It said the boat crashed into a reef after being hit by big waves and strong wind, and sank around dawn. Around 40 people were rescued; emergency teams have been looking for the rest.

Many Indonesians work in Malaysian factories and plantations. Boats are the main mode of transport among the Indonesian archipelago of some 17,000 islands.