A midair collision between two helicopters has killed 13 French soldiers fighting against Islamic extremists in Mali, French President Emmanuel Macron said Tuesday. Macron expressed "deep sadness" at the news of the Monday evening crash, which took place during a combat operation, the AP reports. In a written statement, he expressed his support for the French military and stressed the "courage of the French soldiers" fighting the persistent Islamic threat in the Sahel region. The statement provided no further details about the accident. The operation in West and Central Africa involves 4,500 personnel and is France's largest overseas military mission. The BBC reports that Monday's crash was the French military's worst loss of life in decades. (More Mali stories.)