An affiliate of the so-called Islamic State (ISIL) claimed responsibility Thursday for a helicopter collision that killed 13 French soldiers earlier this week in Mali, while France said it will reassess its military operation in West and Central Africa after its deadliest toll in nearly four decades.

The so-called Islamic State in the Greater Sahara statement, with no evidence, came almost three days after the low-flying helicopters collided on a moonless night while pursuing extremists near the border with Niger. An investigation has begun into the cause of the crash and the flight data recorders have been found.

French military spokesman Col. Frederic Barbry said the military would not comment on the claim. Shortly after the crash, Army Chief of Staff Gen. Francois Lecointre said the helicopters had been supporting French forces on the ground pursuing fighters with the ISIL affiliate.

A national memorial ceremony will take place Monday in Paris.

The helicopter collision drew global attention to an emerging front for ISIL-linked groups as ISIL loses strength in its core area in Syria and Iraq.

France intervened in Mali in 2013 after extremists seized control of major towns in the north and implemented a harsh version of Islamic law. They were forced back into the desert, where they have regrouped.

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

Public frustration in Mali over the insecurity has been directed at the French military presence as well.

The crash was France’s highest military toll since 1983, when 58 paratroopers were killed in a truck bombing in Lebanon.