Thirteen French soldiers were killed in Mali when two helicopters collided while fighting insurgents in the country’s restive north, officials said yesterday, the heaviest single loss for the French military in nearly four decades.

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.