Three Chadian troops and a female civilian were killed early Thursday when jihadists attacked a military position on an island in Lake Chad, the military said.
“We neutralised 21” assailants, armed forces chief of staff General Taher Erda told AFP.
He attributed the attack to the Nigerian-based Boko Haram.
“Our forces are currently sweeping the area looking for Boko Haram elements which were able to flee.”
The attack is part of a mounting campaign by jihadists in the vast, marshy Lake Chad area, where the borders of Cameroon, Chad, Niger and Nigeria converge.
Boko Haram launched an insurgency in Nigeria in 2009 before beginning incursions in its neighbouring countries to the east.
A group called ISWAP that is affiliated to the so-called Islamic State and split from Boko Haram in 2016 is notoriously active in the Lake Chad area.
Benin, Cameroon, Chad, Niger and Nigeria have set up a multinational joint task aimed at rolling back jihadists in the region with the help of local self-defence units.
In early January, all 1,200 Chadians in this force who had been deployed in Nigeria were pulled back to the Chadian side of the lake.
On Monday, six troops were killed in an ambush on the island of Tetewa on Lake Chad. Last week, a suicide bomber killed nine civilians in a village in the same province.
On Wednesday, five civilians were killed at a village in northern Cameroon in a suspected Boko Haram attack.