The black boxes from two French military helicopters that collided in Mali killing 13 soldiers have been found, a French military spokesman said Wednesday.

The crash occurred late Monday during an operation against jihadists in the Liptako region, near the borders with Burkina Faso and Niger. It was the heaviest single loss for the French military in nearly four decades.

"The two black boxes from the helicopters have been recovered, they will be handed over to the relevant authorities to be analyzed," the spokesman, Colonel Frederic Barbry, told BFMTV.

Three helicopters and a squadron of Mirage jets had arrived on Monday to support ground troops pursuing Islamist extremists.

Shortly after troops engaged the insurgents, who fled on motorbikes and in a pickup truck, a Tiger attack helicopter collided with a Cougar military transport helicopter.

All 13 onboard the two aircraft were killed.

Barbry said no theory as to the cause of the crash was being ruled out.

The conditions for flying at the time of the crash were "extremely difficult" because it was a dark night with no moonlight, the spokesman said.

"The pilots work with night-vision binoculars that intensify the residual light when there is no moon, no sources of artificial light like in cities, as is the case in this region."

Barbry said the soldiers' bodies will be repatriated to the French mainland.

In the southwestern French town of Pau, home to a helicopter unit that lost seven troops in the crash, several hundred army veterans, serving officers, officials, and ordinary citizens gathered Tuesday evening to pay tribute to the men.

The accident brought to 41 the number of French troops killed in the Sahel region since Paris intervened against jihadists in northern Mali in 2013.

Since then, armed groups affiliated with the Islamic State group, Al-Qaeda and others have advanced into southern Mali as well as into neighboring Burkina Faso and Niger.

The chief of staff of the French armed forces, General Francois Lecointre, warned Wednesday against expecting total victory over insurgents roaming an area the size of Western Europe.

"We will never achieve a definitive victory," he told France Inter radio while insisting that France's intervention was "useful, good and necessary".

"We are producing results but we must be patient and persevere," he said, adding that a lasting solution to the unrest in the region required "military action but also action on the development front."

France has 4,500 troops deployed to help local forces hunt jihadists in Burkina Faso, Chad, Mali, Mauritania, and Niger.

It had been hoping that a joint counter-terrorism force set up by the five African countries would gradually take over the operations.

But the G5 Sahel force has been plagued by a lack of manpower, funds, training, and weaponry.

Regional armies have also suffered heavy losses in the fighting.

In some of the deadliest incidents, 43 Malian soldiers were killed in an attack in the east of the country in mid-November whereas Burkina Faso lost 24 troops in an assault on a base near the Malian border in August.

The UN's 13,000-strong peacekeeping mission in Mali, MINUSMA, has also regularly come under attack, losing over 200 troops since 2013.

Despite the challenges and the growing hostility towards French troops in Mali and Burkina Faso, both former colonies, President Emmanuel Macron's government is adamant it has no plans to scale back operations.

France has presented the battle against the jihadists operating on Europe's doorstep as a battle for the security both of Africa and Europe.

On Tuesday, outgoing EU Commission chief Jean-Claude Juncker also held that line, saying that "in Mali as elsewhere, it is the French army that is defending Europe's security and honor."