French presidency says crash of two helicopters took place during an operation against fighters in northern Mali.

Thirteen French soldiers have been killed in a collision between two helicopters in Mali, in what is believed to be the singlest biggest loss of the European country’s troops there since its intervention in 2013.

The accident occurred on Monday evening while the soldiers were engaging fighters who have staged a series of deadly attacks in northern Mali in recent weeks, French President Emmanuel Macron‘s office said.

“The president announces with deep sadness the death of 13 French troops in Mali on the evening of Nov. 25, in an accident between their two helicopters during a combat mission against jihadists,” the statement on Tuesday said.

An inquiry has been opened into the cause of the mid-air collision, the defence minister, Florence Parly said in a separate statement.

Defence ministry sources told AFP news agency a Tiger attack helicopter collided with a larger Cougar military transport helicopter.

Six officers and a master corporal were among the victims.

The accident brings to 38 the number of French soldiers killed in Mali since France’s intervention six years ago to help drive back armed groups threatening to overrun the country.

It has since kept about 4,500 troops in the region as part of its Barkhane operation, which is primarily tasked with building up and training local security forces but also participates in operations against armed groups.

Western powers have also provided funding to a regional force made up of soldiers from Mali, Niger, Burkina Faso, Chad and Mauritania to combat armed groups, but the so-called G5 force has been hobbled by delays in disbursing the money and poor coordination.

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