A Eurocopter Tiger (Eurocopter EC665 Tigre) helicopter (left) is seen at the French Military base in Gao, in northern Mali on November 8, 2019. Photo: VCG

The black boxes from two French military helicopters that collided in Mali killing 13 soldiers have been found, a French military spokesperson said Wednesday.

The crash occurred late Monday during an operation against jihadists 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 analyzed," the spokesperson, Colonel Frederic Barbry, told BFMTV.

Three helicopters and a squadron of Mirage jets arrived 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 onboard the two aircraft.

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 spokesperson 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."

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