Up to 600 people were shot dead in a matter of hours by al Qaeda-linked militants in an August attack on a town in Burkina Faso, according to a French government security assessment that nearly doubles the death toll cited in earlier reports. The new figure would make the assault, in which civilians were shot dead as they dug trenches to defend the remote town of Barsalogho, one of the deadliest single attacks in Africa in recent decades.

Militants from Jama’at Nusrat al-Islam wal-Muslimin (JNIM), an al Qaeda affiliate based in Mali and active in Burkina Faso, opened fire methodically as they swept into the outskirts of Barsalogho on motorcycles and shot down villagers, who lay helpless in the freshly upturned dirt of the trench, according to several videos of the August 24 attack posted by pro-JNIM accounts on social media. Many of the dead were women and children, and the footage is punctuated by the sound of automatic gunfire and screams of victims as they are shot while apparently trying to play dead.

The horrific death toll, if the French government estimate is confirmed, would mark an unusually brutal moment in the Sahel, an increasingly lawless swathe of West Africa just south of the Sahara where security projects spearheaded by the United States and French militaries have struggled to slow the march of jihadists. A series of coups across Mali, Burkina Faso and Niger led to the departure of French and American forces. The Russian mercenaries summoned by the juntas to bolster their grip have instead left a vacuum in which jihadists have thrived, says the assessment, given to CNN by a French security official.