Three suicide bombers attacked the island of Koulfoua on the Chadian side of Lake Chad on Saturday, killing around 30 people and injuring 80, two security sources told Reuters.
The sources said the attacks coincided with market day. It was not immediately clear who was responsible for the attacks although the swampy maze of islands has previously been targeted by Islamist militant group Boko Haram from neighbouring Nigeria.
Chad extended a state of emergency in the region last month after a double suicide attack that killed 12 people.
Earlier this year, thousands of people fleeing Boko Haram sought refuge on the island although it was not immediately clear if they were still based there.
The U.N. Office for the Coordination of Humanitarian Affairs said this week that over 50,000 internally displaced people had arrived in the region since end-July.