Boko Haram Islamists have attacked Chadian soldiers, killing 11 and wounding 13 in a raid near Lake Chad.
The source said 17 Boko Haram fighters also died in the fighting following the pre-dawn strike.
"Boko Haram members attacked our positions at 4:30am in Kaiga Ngouboua about two kilometres from the Nigerian border," a Chadian security source told AFP.
"This surprise attack claimed the lives of 11 soldiers and wounded 13 [and] 17 Boko Haram fighters were killed.
"The attackers were pushed back and the army is continuing search operations in the zone."
Chad has joined a regional military alliance, alongside Niger, Cameroon and Nigeria, to fight Boko Haram, which has waged a bloody insurgency since 2009 marked by mass abductions, village massacres and suicide bombings by women and teenagers.
At least 17,000 people have been killed and more than 2.5 million made homeless since the Boko Haram insurgency began in 2009.