Thirteen French troops were killed when two helicopters collided during an operation against terrorists in Mali.
A statement from the presidential palace said the accident happened on Monday evening.
French President Emmanuel Macron expressed his “deep sadness” over the incident.
In 2013, France deployed thousands of troops to Mali after militants seized huge parts of the north.
Mali’s army has since recaptured the territory but insecurity there continues and the violence has spread to other countries in the region.
France now has 4,500 troops deployed to support the forces of Mali, Mauritania, Niger, Burkina Faso and Chad against militants.
Another French soldier, Brigadier Ronan Pointeau, was killed earlier this month after an explosive device detonated near his vehicle.
Monday’s incident is thought to be the worst loss of French life since the country first intervened in Mali.
In total 38 French soldiers have been killed in Mali since 2013.
“These 13 heroes had only one goal: to protect us,” President Macron wrote in a tweet. “I bow to the pain of their loved ones and comrades.”