Seven soldiers were also injured in Thursday's ambush, Sadick Khatir said, confirming information given by an anonymous military source.

Contacted by AFP, however, army spokesman Azem Mbermandoa, did not confirm the death toll but added that the Chadian army had "destroyed a Boko Haram base, recovered weapons and ammunition".

The jihadist group, which originated in Nigeria in 2009, has established bases on islets dotting Lake Chad, a vast swampy expanse on the border between Nigeria, Chad, Niger and Cameroon.

The Chadian army launched an offensive against Boko Haram in April after the death of some 100 soldiers in an attack by the group on one of its bases.

President Idriss Deby then claimed to have pushed the jihadists out.

But despite this military operation, the attacks have continued.

In Chad's Lake Province, more than 360,000 people have fled their homes to avoid attacks and also flooding, according to the International Organization for Migration (IOM).

The president admitted in early August that "Boko Haram would still do a lot of damage" in Chad.