Two helicopters full of French troops pursuing militants in a remote region of Mali collided on Monday evening, killing 13 soldiers.

It's the "heaviest single loss for the French military in nearly four decades," according to Agence France-Presse. In 1983, an attack in Beirut killed 58 French paratroopers.

"These 13 heroes had only one goal: to protect us," French President Emmanuel Macron said in a tweet. "I bow my head in front of the pain of their families and comrades."

"The loss is heavy but the peoples of the Sahel share your bereavement," said Mali's president, Ibrahim Boubacar Keita, according to the BBC. The Sahel is a vast desert region of northwest Africa that includes Mali.

While it's not clear which group the militants belong to, Army Chief of Staff Gen. François Lecointre said at a news conference that an affiliate of ISIS was the main group in the area, AFP reported.

The helicopters were brought in to back up ground commandoes who were pursuing militants near Mali's border with Burkina Faso.

France's defense minister, Florence Parly, told reporters that the helicopters were flying "in total darkness, which made the operation much more complicated," according to The Associated Press.

"Two explosions were heard and the aircraft hit the ground a short distance apart. There were no survivors," Reuters reported, citing French officials.

The French government has reportedly recovered the choppers' flight data recorders and opened an investigation to pinpoint how the collision happened.

French troops intervened in Mali, a former French colony, in 2013 after radical jihadists took over parts of the country. At least 44 French troops have died since then, as The Associated Press reported. In recent years, France has taken a combat role in several other African countries, including the Central African Republic and Ivory Coast. About 4,500 French troops are currently deployed across five African countries, according to the BBC.

Reports from the region suggest that the security situation has grown worse in this area that France is trying to stabilize.

In the past two months, Mali has seen a surge of militant attacks — as the AP reported, "well over 100 local troops" have been killed, and IS has claimed responsibility for many of them.

In a recent newspaper op-ed, former French diplomat Bruno Clément-Bollée stated that "in the Sahel, France has a lot to be worried about." In the piece quoted by the BBC, he said: "We seem to have no idea how to get out of the quagmire."

The French government has struggled to obtain more troop support from the European Union to aid in this fight. Parly, the defense minister, said that "this is not the time for questioning the merit or not of this operation," according to AFP. "It's a time for mourning."