N'DJAMENA - Suspected militants from Nigeria's Boko Haram group killed 11 soldiers in an attack over the border in Chad early on Tuesday, a Chadian military source said.
Seventeen attackers also died in the clash in an area that has seen a series of raids by the fighters, increasingly moving out of their heartlands to strike in Nigeria's neighbors.
Chad is a leading contributor to a 8,700-strong regional force fighting Boko Haram, a movement which wants to carve out an Islamist caliphate and has allied itself to Islamic State.
"It was around 4 a.m. (3:00 a.m. GMT) that they attacked us by surprise," an officer involved in the battle in the Lake Chad border region told Reuters.
Chad's President Idriss Deby has said he believes Boko Haram can be defeated by the end of the year.
His capital N'Djamena is less than 100 km (60 miles) from the group's stronghold in northeastern Nigeria and will be the regional force's headquarters.