Thirteen French soldiers were killed in Mali when their helicopters collided at low altitude as they swooped in at nightfall to support ground forces engaged in combat with Islamist militants.
It was the biggest loss of French troops in a single day since an attack in Beirut 36 years ago when 58 soldiers died.
The ground commandos had been tracking a band of militants moving on pick-up trucks and motorbikes.
After identifying the group yesterday, the Tiger and Cougar helicopters were sent to reinforce, along with a combat jet.
The collision occurred at 7.40pm local time, when darkness would have fallen over the area, as the helicopters manoeuvred ahead of engaging with the militants, the army said.
The Tiger attack-helicopter and multipurpose Cougar hit the ground a short distance apart. There were no survivors.
French President Emmanuel Macron expressed his "deep sadness" at the loss.
"The President of the Republic salutes with the greatest respect the memory of these soldiers," the Elysee Palace said in a statement.
"He bows to the grief of their families and their loved ones."
France, the former colonial power in the region, first intervened in Mali in 2013 to drive out militants who had occupied the north.
It still has a 4,500-strong force countering insurgencies in the wider region. However, rather than stabilising, security has progressively worsened.
Islamist militants with links to al Qaeda and Islamic State have strengthened their foothold across the arid region, making large swathes of territory ungovernable and stoking ethnic violence, especially in Mali and Burkina Faso.
Two years ago, four US special forces died in a firefight with militants in Niger, an incident which shone a light on the US counter-terrorism activity in the West African Sahel, which lies just below the Sahara desert.
In a message of condolence to Mr Macron, Malian President Ibrahim Boubacar Keita said the French soldiers "died for Mali, they died for the Sahel, they died for freedom".
Today's deaths bring the total number of French soldiers killed in the Sahel region since 2013 to at least 38, officials said.
More than 200 soldiers from regional nations and international peacekeepers have been killed since September in Mali alone, with dozens more killed in Burkina Faso.
Irish troops form part of the UN peacekeeping mission in the region.