Two helicopters assisting French soldiers in their pursuit of Islamic extremists collided in Mali and killed 13 men, in what is France’s single biggest loss of life since they began counterterrorism operations in the West African country in 2013.

The French military said Tuesday both helicopters were flying very low Monday evening when they collided and crashed in Mali’s Liptako region while supporting French commandos on the ground. No one on either helicopter survived the collision, which is now under investigation.

“These 13 heroes had only one goal: protecting us,” French President Emmanuel Macron tweeted. “I bow to the pain of their loved ones and comrades.”

In this 2017 file photo, a French soldier stands inside a military helicopter during a visit by French President Emmanuel Macron to the troops of Operation Barkhane, France's largest overseas military operation, in Gao, northern Mali. (AP)

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France’s operation in West and Central Africa is its largest overseas military mission and involves 4,500 personnel, according to the Associated Press. France intervened in 2013 after extremists seized control of major towns in northern Mali and implemented a harsh version of Islamic law. They were forced back into the desert, where they have regrouped and moved south into more populated areas.

Attackers linked to the Islamic State or Al Qaeda this month alone, the AP says, have killed scores of local troops in the region and ambushed a convoy carrying employees of a Canadian mining company, leaving at least 37 dead. Since 2013, at least 44 French soldiers have died in the mission.

A new surge in extremist attacks in Mali also has killed well over 100 local troops in the past two months, with ISIS often claiming responsibility. The extremists loot military posts and profit from mining operations while finding refuge in forested border areas. As a result, hundreds of thousands of civilians have been forced to flee their homes.

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Before his death this year, ISIS leader Abu Bakr al-Baghdadi congratulated "brothers" in Mali and neighboring Burkina Faso for pledging allegiance.

Public outrage in Mali also has been directed in recent weeks against France, the country’s former colonizer, over the failure to stop the violence that has led to deadly clashes between wary communities amid suspicions of supporting the extremists.

Mali’s President Ibrahim Boubacar Keita on Tuesday sent his “deepest condolences” to France.

“The loss is heavy but the people of the Sahel share your mourning,” he said in a statement.

Mali’s Liptako region near the border with Niger and its Gourma region near the Burkina Faso border have become strategic crossings for extremist groups as they are largely unguarded, the International Institute for Strategic Studies (IISS) wrote last month.

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France’s operation became involved in the Liptako area in 2017 and this year it built a new base in Gossi in the Gourma region, IISS said.

“Despite increased French presence in this zone, military gains remain limited. Both sides barely ever engage in direct confrontation. Militants use guerrilla tactics, rely heavily on improvised explosive devices and hide within the civilian population before and after launching attacks,” it added.

The Associated Press contributed to this report.