The accident occurred late Monday when three helicopters and a squadron of Mirage jets arrived to support ground troops pursuing Islamist extremists in the Liptako region, near the borders of Burkina Faso and Niger, the defence ministry said.

The accident brought to 41 the number of French troops killed in the Sahel region since Paris intervened against al-Qaeda-affiliated jihadists in Mali in 2013.

It also underscored the challenge for France of trying to control an insurgency in an area the size of Western Europe as a time of escalating violence.

Defence Ministry Florence Parly said Monday's operation was particularly risky because it took place in "total darkness" on a moonless night, but the exact cause of the collision remains unknown.

Shortly after troops engaged the insurgents, who fled on motorbikes and in a pick-up truck, a Tiger attack helicopter collided with a Cougar military transport helicopter, killing all onboard the two aircraft.

Mali has been besieged by militants carrying out deadly strikes against army bases and other targets in recent weeks, a flare-up of violence despite years of efforts to stem the attacks.

Parly told reporters President Emmanuel Macron would lead a commemorative service for the soldiers at the Invalides military hospital and museum in Paris "in the coming days".

General Francois Lecointre, speaking alongside Parly, said the "black box" flight data recorders had been recovered from both aircraft.

He did not identify the insurgents, but said the Islamic State in the Greater Sahara (ISGS) -- an affiliate of the group active in Iraq and Syria -- was the main rebel force in the area.

- 'To protect us' -

One of the victims Monday was the son of French senator Jean-Marie Bockel, a centrist former government minister who sits on the senate's armed forces committee, the father confirmed to AFP.

"These 13 heroes had just one goal: To protect us. I bow my head in front of the pain of their families and comrades," Macron said on Twitter.

Mali's President Ibrahim Boubacar Keita said the soldiers had died for Mali and all Sahel countries, as well as for France.

"The loss is heavy but the peoples of the Sahel share your bereavement," he said in a letter to Macron.

France's 4,500-member Barkhane force in Mali, Burkina Faso, Chad, Niger and Mauritania, is tasked with building up and training local security forces but also participates in operations against the insurgents.

Yet French officials acknowledge that local security forces remain woefully under-equipped and under-financed for shouldering the anti-jihadist fight despite years of French engagement.

And while Britain has provided helicopters and security personnel, and the US offers intelligence support and funding, Paris has failed to convince EU allies to contribute significant troops to the Sahel fight.

- Indispensable -

The worsening security situation has raised fears of France becoming bogged down in a fight that cannot be won solely with military means.

While most parties back the Barkhane operation, the far-left France Unbowed party called on the government Tuesday to open a "serious and rational discussion to consider the ways out of this war".

But Prime Minister Edouard Philippe insisted that the military deployment was "indispensable" to disrupt militant groups and ensure political stabilisation and economic development.

German Chancellor Angela Merkel offered her "sincere condolences" to the soldiers' families, while Ursula von der Leyen, the incoming EU Commission chief, said "we share the pain of all French people".

The accident was the heaviest loss for the French army since the 1983 attack on the barracks of a multinational force in Beirut during Lebanon's civil war, which claimed the lives of 58 paratroopers as well as 241 Americans.

It comes after France announced the death last month of Ali Maychou, a Moroccan leader of the Group to Support Islam and Muslims (GSIM), considered the top jihadist leader in Mali.

Since January, more than 1,500 civilians have been killed in jihadist violence in Mali and Burkina Faso, and more than one million people have been internally displaced across the five countries, the UN said this month.

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