PARIS (AFP) - The black boxes from two French military helicopters that collided in Mali killing 13 soldiers have been found, a French military spokesman said on Wednesday (Nov 27).

The crash occurred late on Monday during an operation against Islamists in the Liptako region, near the borders with Burkina Faso and Niger. It was the heaviest single loss for the French military in nearly four decades.

"The two black boxes from the helicopters have been recovered, they will be handed over to the relevant authorities to be analysed," the spokesman, Colonel Frederic Barbry, told BFMTV.

Three helicopters and a squadron of Mirage jets had arrived on Monday to support ground troops pursuing Islamist extremists.

Shortly after troops engaged the insurgents, who fled on motorbikes and in a pickup truck, a Tiger attack helicopter collided with a Cougar military transport helicopter, killing all 13 on board the two aircraft.

Col Barbry said no theory as to the cause of the crash is being ruled out, and it would be up to the investigation team to determine why the collision occurred.

The conditions for flying at the time of the crash were "extremely difficult" because it was a dark night with no moonlight, the spokesman said.

"The pilots work with night vision binoculars that intensify the residual light when there is no moon, no sources of artificial light like in cities, as is the case in this region."

Col Barbry said the soldiers' bodies will be repatriated to the French mainland.

The accident brought to 41 the number of French troops killed in the Sahel region since Paris intervened against Al-Qaeda-affiliated militants 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.