SAHARA DESERT, Mali: Under the cover of large acacia trees, the militants’ camp was invisible from the sky and had probably been deserted only a few hours before the arrival of French troops.

The soldiers had made the discovery on day two of Operation Gustav, a hunt for rebel fighters in a valley in northern Mali and one of France’s largest military operations during its three-month intervention in its former colony.

The soldiers of the 92nd Infantry Regiment advanced tentatively on foot in battle formation ahead of several tanks, uncovering weapons, the remnants of bivouacs, camping equipment and fuel drums.

A thousand French troops backed by tanks and covered from the air had surrounded the valley near the city of Gao, thought to be a logistics base of the MUJAO rebels, at dawn on Sunday.

Operation Gustav comes with France preparing to withdraw three-quarters of the 4,000 troops it deployed in January to block a feared advance on the Malian capital Bamako by Al-Qaeda-linked insurgents.

Its intervention has driven the militants from most of their northern strongholds but Paris is preparing to hand over to a UN-mandated African force in the coming weeks, leaving just 1,000 of its troops to fight terrorism.

The Malian military, poorly paid, ill-equipped and badly organised, fell apart last year in the face of an uprising by Tuareg rebels who seized the vast arid north in the chaos which followed a military coup, before losing control to well-armed militants.

The extremists terrorized locals with amputations and executions drawing global condemnation and prompting France’s intervention.

While French-led troops have inflicted severe losses on the militants, soldiers are still battling significant pockets of resistance around Gao, where MUJAO are the most active rebel group.

Two Tiger attack helicopters circled in the sky and watched the ground in front of the infantrymen while tanks and heavy artillery observed from ridges high above the valley floor.

Further on, in a wooden box inscribed in French, the soldiers found an old VHF radio, probably stolen from the Malian army.

“This is rare, a godsend,” said a captain named Jean-Baptiste.

“This will allow our specialists to see which frequencies they use or which they listen to.”

The officer said the men had come across ten white pick-up trucks in less than 20 hours and was sure militants were still nearby.

“Maybe they’re watching us. They’re very flexible,” he added.

“They avoid us when they see that we are strong, that they have no chance, but when they spot a vulnerability, they can attack in numbers and show themselves to be a formidable enemy. We have to be super-vigilant.”

Under a perfect rotunda formed by the branches of a giant acacia not far from the camp, the soldiers discover six 122mm rockets, four metres long, next to a truck battery.

“You just have to plug in the bottom to make the rocket go. They make improvised launch ramps with boards and rocks,” says Eric, one of the engineers.