A deadly mid-air collision between two French military helicopters has turned into the worst loss of life for the French military in nearly four decades, since the death of 58 French paratroopers in an attack in Beirut, Lebanon in 1983.

French President Emmanuel Macron on Tuesday expressed “deep sadness” at the news of Monday’s evening crash, which took place during a combat operation against jihadist fighters in Mali.

Macron, in a written statement, reiterated his support for the French military and saluted the “courage of the French soldiers” fighting the Islamic threat in the Sahel region. “These 13 heroes had just one goal: To protect us. I bow my head in front of the pain of their families and comrades,” Macron posted on Twitter.

The French Armed Forces Ministry released a statement saying that the accident occurred late Monday during a counter-terror operation. The choppers were providing transport and close air support for their ground troops battling Islamic insurgents in the Liptako region, near the borders of Burkina Faso and Niger. It is believed that the insurgents targeted in this operation were affiliated with ISIS.