(CNN) Thirteen French soldiers have died in an accident involving two helicopters during an operation against jihadists in Mali, French President Emmanuel Macron's office said Tuesday.

The accident involving six officers, six non-commissioned officers and a master corporal happened Monday during a combat mission, according to a statement from Elysee Palace.

Macron expressed "deep sadness" over the incident.

"They died for France in the hard fight against terrorism in the Sahel. He sends their families and their loved ones his most sincere condolences, assuring them of the unfailing solidarity of the nation," the statement said.

The deaths are the single-biggest loss for the French army since the 1983 Beirut barracks bombing, when 58 paratroopers were killed, according to a spokesman for the French Defence Ministry.

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