* New military base to be set up in northern Niger
* France wants to disrupt militants regrouping in Libya
* Five men linked to Islamists captured in recent weeks (Adds detail, reaction from Niger)
By John Irish
PARIS, Oct 2 (Reuters) - France is setting up a base in northern Niger as part of an operation aimed at stopping al Qaeda-linked militants from crisscrossing the Sahel-Sahara region between southern Libya and Mauritania, officials said.
Paris, which has led efforts to push back Islamists in the region since intervening in its former colony Mali last year, redeployed troops across West Africa earlier this year to form a counter-terrorism force.
Under the new plan, about 3,000 French troops are now operating out of Mali, Burkina Faso, Niger and Chad -- countries straddling the vast arid Sahel band -- with the aim of stamping out Islamist fighters across the region. Another 1,000 soldiers are providing logistical support in Gabon and Senegal.
"A base is being set up in northern Niger with the throbbing headache of Libya in mind," a French diplomat said.
Neither France nor Niger has said where the base will be but military sources in Niger said it was likely to be around Madama, a remote desert outpost in the northeast, where Niger already has some troops based.
French officials have repeated for several months they are concerned by events in Libya, warning that the political void in the north is creating favourable conditions for Islamist groups to regroup in the barren south of the country.
Diplomatic sources estimate about 300 fighters linked to al Qaeda's North African arm AQIM, including a splinter group formed by veteran Islamist commander Mokhtar Belmokhtar, are operating in southern Libya, a key point on smuggling and trafficking routes across the region.
French and American drones are already operating out of Niger's capital Niamey.
Echoing the French push to get assets closer to Libya, U.S. officials said last month that the United States was preparing to possibly redeploy its drones to Agadez, some 750 km (460 miles) to the northeast.
Three years after they launched air strikes to help topple Muammar Gaddafi, Western powers including France have ruled out military intervention in Libya, fearing that it could further destabilise the situation given that countries across the region are backing different political and armed groups in Libya.
However, with France particularly exposed in the Sahel-Sahara region and its forces now engaged in a support role against Islamic State militants in Iraq, Paris is stepping up efforts to squeeze militants in the area.
The murder of a French citizen last week in neighbouring Algeria by former AQIM militants who pledged allegiance to Islamic State also appears to have toughened Paris' resolve.
"The approach to (fighting terrorism) is global," Army spokesman Gilles Jaron said on Thursday. "We are on the frontline in the Sahel-Sahara region and supporting in Iraq."
The French operation, dubbed Barkhane after the name of a kind of sand dune formed by desert winds, has set up its headquarters in the Chadian capital N'Djamena, but also placed an outpost in northern Chad about 200 km from the Libyan border.
Jaron said the new Niger base was still being finalised, but would have capacity for as many as 200 soldiers with aerial support. "The aim is to bring together areas that interest us. The transit points which terrorists are likely to use," he said.
There have been some successes in recent weeks. Two diplomatic sources said Abou Aassim El-Mouhajir, a spokesman for Belmokhtar's "Those Who Sign in Blood" brigade, was captured by French troops in August.
French media said he had been taken in Niger. Niger intelligence sources said French troops had passed through Madama around the time of the operation.
Jaron said four suspected militants were also captured on Sept. 24 near Gao in northern Mali, where France has handed the bulk of security control to U.N. MINUSMA peacekeeping forces.
At the same time there has been an increase in attacks on foreign troops in Mali, including the death of 10 Chadian soldiers in September.
The U.N.'s peacekeeping chief, Herve Ladsous, said last week that with many French troops leaving the north of Mali, U.N. forces were being targeted and finding it difficult to respond due to a lack of helicopters and special forces.
"It's a problem that is being resolved. We want the MINUSMA to be up to scratch so we can focus on our number one job: getting rid of AQIM," said a French defence ministry source. (Additional reporting by David Lewis in Dakar and Abdoulaye Massalaki in Niamey; Editing by Robin Pomeroy)
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