France was in mourning on Tuesday after 13 soldiers from its anti-terrorist Barkhane force in Mali were killed in a helicopter collision during an operation in the country's north. The Elysee Palace said the incident happened at nightfall on Monday when the forces were fighting terrorists, who have staged a series of deadly strikes in the country in recent weeks. Two helicopters were flying very low when they collided and crashed in the northern region of Liptako while supporting French soldiers on the ground pursuing a group of extremists. Among the dead were six officers and a master corporal. No one on board the helicopters survived. One of the victims was the son of French Senator Jean-Marie Bockel, a centrist former government minister. France has more than 4,500 troops in Africa's Sahel region, which also includes Niger, Burkina Faso and Chad. The troops are tasked with training up local security forces but also take part in operations, countering violence from militants linked to Al Qaeda and ISIS. Monday's incident was the deadliest attack since the French intervention began in 2013. In total, 38 French soldiers have been killed in the region. The crash has resulted in the worst loss of life for the French army since 1983, when 58 French military personnel were killed by two truck bombs in Beirut. France's President Emmanuel Macron expressed his “deep sadness” at the news. He praised the “courage of the French soldiers” in a written statement released on Tuesday. "These 13 heroes had only one goal: to protect us," Mr Macron wrote on Twitter. "I bow to the pain of their loved ones and comrades." France's Defence Minister Florence Parly said in a separate statement that an inquiry would be launched into the cause of the collision. She said the country would not turn its back on fighting terrorism in the region. "We receive valuable support from our European allies and join forces to protect Europe from the scourge of terrorism", Ms Parly told a press conference on Tuesday afternoon. "We stand tall, united, resilient. It is this solidarity which strengthens us, which allows us to continue the fight," she added. The minister confirmed that a Tiger attack helicopter had collided with a larger Cougar military transport helicopter during the operation which was conducted in darkness. General Francois Lecointre said the "black box", which record flight data from both the helicopters. It was not clear which militants the French were fighting but Gen Lecointre said a Saharan affiliate of ISIS was the main insurgent force in the area. Mali's government spokesman Yaya Sangare extended his condolences to the family of the soldiers killed by phone from the country's capital, Bamako, on Tuesday. “France and its military operation Barkhane have been crucial for the fight against terrorists in Mali and to re-establish the sovereignty of Mali’s territory,” he said in comments reported by Bloomberg. France's involvement in the region has so far generated little debate at home and most political parties back the intervention. However, leftist party Unbowed said on Tuesday the casualties should lead the government to having a "serious and rational discussion to consider the ways out of this war". But minister Ms Parly said it was not "the time for questioning the merit or not of this operation. It's a time for mourning". Both German Chancellor Angela Merkel and the European Union's Commission chief Ursula von der Leyen paid their respects to the victims. "As a former Defence Minister, I know the true value and importance of the work French troops are doing to fight terrorism, for the people of Mali as much as for the people of Europe," Ms von der Leyen wrote on Twitter. Earlier this month, UN Secretary General Antonio Guterres warned that terrorist groups have strengthened their foothold across the Sahel region. Over 100 local troops have been killed in extremist attacks, the majority of which ISIS has claimed responsibility for. Abu Bakr Al Baghdadi praised his “brothers” in Mali and Burkina Faso for pledging allegiance to ISIS before his death this year.