French president Emmanuel Macron has said a mid-air collision between two helicopters has killed 13 French soldiers fighting against Islamic extremists in Mali.
Mr Macron expressed “deep sadness” after the Monday evening crash.
“These 13 heroes had only one goal: protecting us,” he tweeted.
It is thought to be France’s highest military death toll in nearly four decades.
The military said the helicopters were flying very low when they collided and crashed in Mali’s Liptako region near Niger while supporting French commandos on the ground pursuing a group of extremists. No one on board survived.
Treize de nos militaires sont morts hier au Mali. Ils étaient engagés dans une opération de combat contre des terroristes. Ces treize héros n’avaient qu’un seul but : nous protéger. Je m’incline devant la douleur de leurs proches et de leurs camarades.
— Emmanuel Macron (@EmmanuelMacron) November 26, 2019
French defence minister Florence Parly said the helicopters were operating “in the total darkness, which made the operation much more complicated”.
The helicopters’ flight data recorders have been found and an investigation has begun.
France’s operation in West and Central Africa is its largest overseas military mission and involves 4,500 personnel.
France intervened in 2013 after extremists seized control of major towns in northern Mali and implemented a harsh version of Islamic law.
They were forced back into the desert, where they have regrouped.
Since 2013, at least 44 French soldiers have died in the Sahel.
Mali’s president Ibrahim Boubacar Keita on Tuesday sent his “deepest condolences” to France.
“The loss is heavy but the people of the Sahel share your mourning,” he said in a statement.
The region is an emerging front in the fight against the so-called Islamic State group and other extremist groups, including those linked to al Qaida, the United States says.
French politicians observed a minute of silence on Tuesday.
Prime Minister Edouard Philippe defended military operations in the Sahel, saying that “without the ability to face the enemy … we cannot guarantee, on another hand, the essential work of political stabilisation and economic development”.
A new surge in extremist attacks in Mali has killed well over 100 local troops in the past two months, with IS often claiming responsibility.
Before his death this year, IS leader Abu Bakr al-Baghdadi congratulated “brothers” in Mali and neighbouring Burkina Faso for pledging allegiance.
Mali’s Liptako region near the border with Niger and its Gourma region near the Burkina Faso border have become strategic crossings for extremist groups as they are largely unguarded, the International Institute for Strategic Studies (IISS) wrote last month.
France’s operation became involved in the Liptako area in 2017 and this year it built a new base in Gossi in the Gourma region, IISS said.
“Despite increased French presence in this zone, military gains remain limited. Both sides barely ever engage in direct confrontation.
“Militants use guerrilla tactics, rely heavily on improvised explosive devices and hide within the civilian population before and after launching attacks,” it added.
France’s Barkhane military operation is one of multiple efforts against the growing extremist threat in the Sahel including a five-nation regional counterterror force that struggles to secure international funding and a UN peacekeeping mission in Mali.