Chadian soldiers on Wednesday repelled an incursion into a volatile northern gold-mining region from neighbouring Libya, the army said.
The rebel group, the Military Command Council for the Salvation of the Republic (CCMSR), acknowledged the deadly fighting, saying its forces had carried out a "strategic pullback".
The clash took place early Wednesday near Kouri Bougoudi, a town hard by the Libyan border in the Tibesti massif, a mountainous gold-mining region where lawlessness and trafficking are rampant.
CCMSR rebels "attacked army positions at 6 am," army spokesperson Colonel Azem Bermendoa Agouna told AFP.
The army "routed them and is currently pursuing them," he said.
A senior army officer, speaking on condition of anonymity, said 40 rebels had been killed and 38 captured, against the loss of three soldiers.
The CCMSR's spokesperson, Ali Saleh Hassaballah, reached by phone from Libreville, told AFP that about 50 soldiers had been killed, against the loss of only three rebels.
The starkly contrasting tolls could not be verified independently, as the zone is barred to journalists.
Hassaballah added: "After more than five hours of heavy fighting, we carried out a strategic pullback of about 30km because we heard that army reinforcements were arriving."
A gold miner in the area, contacted by AFP, confirmed "the rebels were pushed back."
Armed groups opposed to President Idriss Deby Itno, who has ruled Chad with an iron fist for nearly three decades, frequently make cross-border raids from Libya.
The CCMSR, created in 2016, describes itself as a political-military opposition movement with "several thousand" fighters.
Last August, a cross-border raid of this kind prompted the government to launch a military crackdown in the Tibesti and declare a state of emergency there - a measure that it lifted five months later.
In February 2019, another armed group, the Union of Resistance Forces (UFR) led by Deby's nephew Timan Erdimi, entered Chad from Libya in columns of pickups.
France - whose anti-jihadist campaign in the Sahel is strongly supported by Chad - carried out air strikes at N'Djamena's request to halt their advance.