BAMAKO, Mali (AP) — Two helicopters collided in midair and killed 13 French soldiers fighting Islamic extremists in Mali, France said Tuesday, in its biggest loss since its mission in West Africa’s Sahel region began in 2013.

The deaths draw new attention to a worrying front in the global fight against extremism. Attackers linked to the Islamic State or al-Qaida this month alone 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.

French President Emmanuel Macron expressed “deep sadness” after the Monday evening crash. “These 13 heroes had only one goal: protecting us,” he tweeted.

The French military said both helicopters were flying very low when they collided and crashed in Mali’s Liptako region. No one on board survived.

The helicopters were supporting French commandos on the ground who were pursuing a group of extremists. French defense minister Florence Parly said an investigation has been opened into the accident.

France’s operation in West and Central Africa is its largest overseas military mission and involves 4,500 personnel. 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.

Since 2013, at least 44 French soldiers have died.

A new surge in extremist attacks in Mali has killed well over 100 local troops in the past two months, with IS often claiming responsibility. The extremists loot military posts and profit from mining operations while finding refuge in forested border areas.

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

Public outrage in Mali over the latest attacks also has been directed in recent weeks against France, the country’s former colonizer.

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 wrote last month.

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.

France's Barkhane military operation is one of multiple efforts against the growing extremist threat in the Sahel including a five-nation regional counterterror force that struggles to secure international funding and a U.N. peacekeeping mission in Mali.

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Corbet reported from Paris. Associated Press writer Cara Anna in Johannesburg contributed.