Three Boko Haram suicide bombers attacked the island of Koulfoua on the Chadian side of Lake Chad on Saturday, killing around 30 people and injuring 80.
The the attacks coincided with market day, Reuters quoting sources reported.
Thousands of people fleeing the militant group sought refuge on the island last year.
It was not immediately clear if the refugees were still based there.
Boko Haram suicide bombers have struck in recent weeks in all the countries abutting the Lake Chad, including Nigeria, Cameroon and Niger.