Thirteen French soldiers fighting Islamist insurgents in Mali have been killed after two helicopters collided in mid-air.No one survived the crash which happened as the aircraft were flying very low and "in total darkness" - making the operation more complex.This is the deadliest incident involving the French military since 1983, when 58 paratroopers were killed in a truck bombing in Lebanon.The troops who died in Monday's tragedy in the Liptako region near Niger were helping French commandos on the ground hunting down a group of extremists.Image:Soldiers killed in mid-air helicopter collision in MaliFrench President Emmanuel Macron expressed his "deep sadness" and called the soldiers "heroes" who "had only one goal: protecting us".The flight data recorders have been found and an investigation is under way.The soldiers were chasing fighters with the Islamic State in the Greater Sahara, which is linked to IS.A new surge in extremist attacks in Mali has killed more than 100 local troops in the past two months.The region is an emerging front in the fight against IS and other extremist groups, including those linked to al Qaeda, the US has said.Before his death this year, IS leader Abu Bakr al Baghdadi congratulated "brothers" in Mali and neighbouring Burkina Faso for pledging allegiance.The French operation in West and Central Africa is its largest overseas military mission and involves 4,500 personnel.Image:A soldier arrives with flowers at the 5th combat helicopter regiment base in Uzein near Pau, FranceFrance 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.Since 2013, at least 44 French soldiers have died in the operation.