The veteran leader died from wounds sustained while commanding troops fighting a rebel incursion, according to the army, opening a period of uncertainty in Chad, a key strategic ally of the West in the Sahel region of Africa.

"Chad is losing a great soldier and a president who has worked tirelessly for the security of the country and the stability of the region for three decades," the office of President Emmanuel Macron said in statement, hailing Deby as a "courageous friend" of France.

The statement also emphasised France's insistence on the "stability and territorial integrity" of Chad as it faces a push by rebel forces towards its capital, N'Djamena.

Defence Minister Florence Parly praised Deby as an "essential ally in the fight against terrorism in the Sahel" while emphasising that the fight against jihadist insurgents "will not stop".

Deby's son was immediately named transitional leader as head of a military council as both the government and parliament were dissolved, but the army vowed "free and democratic" elections after an 18-month transition period.

The statement by the French presidency underscored "the importance of the transition taking place under peaceful conditions".

There should also be "a spirit of dialogue with all political and civil society actors, and allowing the rapid return to inclusive governance based on civil institutions," it added.

French Foreign Minister Jean-Yves Le Drian said it was important that the transition would lead "after a limited period of time" to the establishment of a civilian and inclusive government to serve Chad's people.

Deby had ruled Chad with an iron fist since taking power on the back of a coup in 1990, but was a key partner in the West's anti-jihadist campaign in the troubled Sahel region, where France's 5,100-strong Barkhane force is deployed.