Picking through the spartan remains of the jihadist camps dotting Inais valley in northern Mali, it is obvious to the French soldiers that their enemy is having a tough time of it. The troops sweeping the arid river bed 100 kilometres (62 miles) north of the Malian city of Gao since Sunday have not encountered a single militant, yet evidence of their primitive lifestyle is everywhere. They take shelter in camps under the largest acacia tress, in the most dense woodland in the 20-kilometre basin, out of sight of the villages whose inhabitants they recruit or terrorise, but never trust. "Their primary goal is to reduce their footprint, make themselves invisible from the sky, because they know a fleet is flying night and day over their heads," said a French lieutenant who will only give his first name, Cyrille. The trawl through the valley, nicknamed Operation Gustav, is one of France's largest military operations during its three-month old intervention in its former colony. One thousand French troops along with Malian security forces are backed by tanks and covered from above by drones, helicopters and spy planes equipped with thermal cameras that detect heat sources even in vegetation. But it is difficult to differentiate from the air between four men around a camp fire and a herd of goats, so the soldiers of France's 92nd Infantry Regiment are on foot patrol under the acacia trees. The shadows of the tallest trees provide ideal cover for bivouacs fashioned by Islamist fighters from the needled branches of acacia trees which are bent together to form opaque walls. A soldier easily could pass a few metres in front of one of these without noticing a thing. Operation Gustav comes with France having begun a phased withdrawal of the 4,000 troops it deployed in January to block a feared advance on the capital Bamako by Al-Qaeda-linked insurgents. The intervention drove the militants from most of their northern strongholds, where they had terrorised locals with amputations and executions performed under their brutal interpretation of sharia Islamic law. While French-led troops have inflicted severe losses on the Islamists, soldiers are still battling significant pockets of resistance around Gao from the Movement for Oneness and Jihad in West Africa. In a makeshift camp, the jihadists have left clues to the simple lifestyle they were leading before fleeing the French-led advance into the valley. Almost all the food -- cartons of "Choco Dream" cakes, "Gusto" spaghetti and "Veronica" powdered milk -- comes from Algeria. There are boxes of dates, canned peeled tomatoes and other ingredients which would have made for an uninspiring lunch in the 40C heat in the shade in the afternoon. Mats for sleeping are laid on the floor beside army-issue folding beds, and a pair of walking shoes in good condition is abandoned nearby. There are water bottles hanging in the trees, wrapped in canvas to keep them cool, old tin pots and cutlery made in China. Ammunition caches discovered by the French, usually shells or high-calibre rockets, are often located away from the camps for safety. Near fires, wooden crates make serviceable if not exactly comfortable chairs. "Chief, chief, here," shouts one of the soldiers, holding up an empty 500-litre (110-gallon) barrel, the smell of petrol acrid in the air. Long ago Islamist militias who roamed Africa's Sahel region realised that the big V8 petrol engines of their Toyota pick-ups gave them a crucial speed advantage over the diesel engines of the same pick-ups used by the region's armies. When they are full, the drums are often buried, making them invisible to troops unless their location is pointed out by a local. Islamist fighters keep track of them by making a note of their GPS points. One of the Tuareg scouts accompanying the French soldiers pokes around the ashes of a camp fire with the tip of his stick. "Less than a week old," he says. A French soldier joins one of his comrades in the shade of a thorn tree. "Five days we've been here. You'd think that they weren't expecting us." In the neighbouring village of Fes en Fes, in the centre of the valley, the few villagers who haven't been living for last few months in a tent behind the dune say they kept their contacts with the Islamist guerrillas to a minimum. "We would never see them during the day. They sent people, often very young, running errands to buy tea and sugar," said one of the villagers, Mohammed. "They are suspicious of everyone, they stop everyone from approaching their camps and they treat strangers in the village as spies," he said. After noting the reference points of ammunition and recovering any that might be of use to the Malian army, the French soldiers gather together all the jihadists' belongings, spray them with petrol and set them on fire.