The accident occurred late Monday while the helicopters were reinforcing ground troops pursuing the insurgents in the Liptako region, near the borders of Burkina Faso and Niger, the armed forces ministry said.

Mali has been besieged by a wave of deadly strikes against army outposts and other targets in recent weeks, a flare-up of violence despite years of efforts to push back the Islamist extremists.

A Tiger attack helicopter collided with a Cougar military transport helicopter while engaging the insurgents fleeing on motorbikes and in pick-up trucks.

Both aircraft crashed not far from each other, killing all on board, the ministry said.

- 'Goal to protect us' -

One of the victims was the son of French Senator Jean-Marie Bockel, a centrist and 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," President Emmanuel Macron said on Twitter.

Macron promised this month new measures "in the coming weeks" to bolster the fight against the Islamic insurgency in the Sahel, after receiving the presidents of Mali, Chad and Niger at the Elysee Palace.

France's 4,500-member Barkhane force in Mali and four other West African countries 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.

Such warnings have given grist to critics who say France risks becoming bogged down in a fight it cannot win without significant new investments in soldiers and material.

- Daunting fight -

The accident, the deadliest since France intervened in Mali in 2013 to drive back an intense Islamic insurgency, brings to 38 the number of French soldiers killed in the country.

It was the heaviest loss for the French army since the 1983 attack on the Drakkar building in Beirut, claiming the lives of 58 paratroopers.

An inquiry has been opened into the cause of the mid-air collision, Defence Minister Florence Parly said.

Mali has sustained a wave of insurgency strikes on army outposts and other targets, with more than 50 killed over just a few days in early November.

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

The GSIM has claimed responsibility for the biggest attacks in the Sahel since its official launch in 2017.

Mali's President Ibrahim Boubacar Keita warned this month that the country's stability was at stake, urging people to rally around the country's besieged armed forces as well as foreign forces, which also include the United Nations' 13,000-strong MINUSMA peacekeeping mission.

It is one of the countries in the Sahel region of Africa that has been caught in the eye of the jihadist storm since 2012, along with Niger, Burkina Faso, Niger and Chad.

Yet funding for their joint G5 Sahel force, which is supposed to take over from the Barkhane operation, has compounded training and equipment shortfalls.

Since January, more than 1,500 civilians have been killed in Burkina Faso and Mali, and more than one million people have been internally displaced across the five countries -- more than twice the number of persons displaced in 2018, the UN said this month.