A triple suicide bombing at a market on an island in Lake Chad on Saturday killed at least 27 people and injured 90 others, said security officials.
The three explosions on Koulfoua were carried out by females, said Chad police spokesman Paul Manga.
Nigeria's Islamic extremist group Boko Haram is suspected, according to Chad's director general of the gendarmerie, Gen. Banyaman Cossingar.
U.S. & World
The Lake Chad region, which straddles the borders of Chad, Cameroon, Niger and Nigeria, has been regularly targeted by the extremists. Chad's government in November imposed a state of emergency in the area.
Two suicide bombings also by women killed at least three people in November in Ngouboua village near Lake Chad. Five coordinated suicide bombings in October killed at least 36 people and wounded some 50 others in the western village of Baga Sola near Lake Chad that is home to thousands of Nigerians who have fled the extremists' violence.
Boko Haram's 6-year uprising has killed some 20,000 people. The Nigeria militants have this year expanded attacks into Cameroon, Chad and Niger, countries contributing troops to a regional force formed to wipe out the extremists. Forces from Nigeria and neighboring Chad earlier this year drove the extremists out of cities and towns in northeaster Nigeria where they had proclaimed an Islamic caliphate.
Troops from Cameroon, Nigeria, Chad and Benin had last week launched operations in the Lake Chad region and the Sambisa Forest, arresting 100 militants, killing 100 others and freeing 900 hostages, according to Cameroon's government.
The bombings in Chad come as Nigeria's intelligence agency says it has arrested nine alleged Boko Haram extremists plotting attacks on Abuja, the capital, over the festive season.
Saturday's statement follows a warning Friday from the U.S. Embassy that extremists may be planning attacks on hotels favored by Westerners.
One of the nine men arrested was carrying out surveillance of a "high-profile hotel," said Nigeria's Department of State Security. It said all nine were detained in the past month and had infiltrated Abuja, the capital in central Nigeria, from the country's northeastern area where most extremist attacks occur.
AP writer Michelle Faul contributed to this story from Lagos, Nigeria. This story has been corrected to show that the correct spelling of the police spokesman is Manga.