* Islamist rebels now holed up in remote Saharan hideouts
* Experts says rebels likely to have cached arms, supplies
* Hidden arsenal may still include multiple rocket launchers
* Second phase of Mali operation seen harder than first
* Fears remain of "wild card" strike by jihadists
By David Lewis and Pascal Fletcher
BAMAKO/DAKAR, Feb 1 (Reuters) - The French or African troops who hunt down the Islamist fighters holed up in the mountains and deserts of northeast Mali may find a resilient enemy capable of fighting back with a concealed arsenal of surprising firepower.
France's initial success in its three-week old intervention in its former colony has gained Paris plaudits at home and abroad as a welcome blow struck against radical jihadists threatening Africa and the West.
Timbuktu and Gao, the main Malian towns held by Islamist insurgents since last year, fell to the French at the weekend, and French troops also seized the airport at Kidal, the last urban bastion abandoned by the rebels.
But the next step in stabilising Mali and pursuing the al Qaeda-allied fighters in their remote desert and mountain bolt-holes near Algeria's border looks like a much tougher task.
It will take longer than a few weeks and likely require a bigger and more international effort than the limited offensive that has so far involved 3,500 French soldiers on the ground, backed by warplanes, helicopters and armoured vehicles.
"Both politically and militarily, now is going to be the hard bit," Gregory Mann, a Mali expert who is associate professor of history at Columbia University, told Reuters.
The Islamist forces are thought to be sheltering north of Kidal in the Adrar des Ifoghas, a vast, rugged mountain buttress that has given sanctuary before to al Qaeda hostage-takers and Saharan traffickers of drugs, people and cigarettes.
They are believed to have weapons, fuel and supplies hidden in caves, tunnels and rock strongholds. These were stashed away before their pell-mell retreat from relentless French air strikes that left a trail of rebel charred vehicles and abandoned arms caches in dusty Niger River and Saharan towns.
"This is where they have the bulk of the stuff hidden," said Rudy Atallah, a former counterterrorism director for Africa at the U.S. Department for Defense. "They have barrels of fuel and weapons. They have been preparing for a long time."
Their preserved arsenal could include heavy machineguns, hand-held rocket launchers and also possibly one or more Grad multiple rocket launchers mounted on vehicles, according to arms experts who have viewed photos and footage of munitions caches abandoned by the rebels in their hasty withdrawal.
"This is pretty heavy ordnance, a level that would achieve parity with or even out gun most West African militaries," James Bevan, head of Conflict Armament Research, told Reuters after viewing photos of a cache found at Diabaly in central Mali.
A Western security source, who asked not to be named, said air power would help in the next phase - but only so much.
"Ground troops will have to go into the mountains and that will lead to casualties," he said.
Guinea's President Alpha Conde, whose West African country is offering troops to a U.N.-backed African intervention force being deployed in Mali, predicted a "battle in the Sahara" against what he called "narco-traffickers" and "terrorists".
"The rebels will not disappear into the sky," he said last week in Davos, Switzerland.
"If we don't want the Sahara to become Afghanistan, then we need the world to get involved, not just France and Africa but also the United States and the European Union," he added.
But the United States and Europe, where a recession-hit public has little appetite for overseas wars after Iraq and Afghanistan, have ruled out sending combat troops and offer instead training as well as logistical and intelligence support.
French President Francois Hollande, anxious to reassure his people France will not get bogged down in a messy war in a faraway former colony, has said he expects African forces to take over the job of hunting down the rebels in the north.
It is hard to know exactly what materiel and manpower the Islamist rebels have lost in hundreds of French air strikes that are now homing in on rebel positions north of Kidal.
A French military video showed a rebel Grad multiple rocket launcher destroyed near Gao.
French estimates speak of several dozen rebels killed in the limited direct clashes so far. The real figure could be higher given the intensity of the air strikes.
Estimates by some security experts had put the combined original strength of the Islamist alliance in northern Mali, which groups al Qaeda's North African wing AQIM, Malian group Ansar Dine and AQIM splinter MUJWA, at around 3,000 fighters.
Mark Schroeder, director of Sub-Saharan Africa analysis at Stratfor consultancy, believes France and its allies will try to prevent rebels from fleeing over Mali's porous Saharan borders into neighbouring states such as Algeria, Niger and Libya.
"If AQIM can be degraded ... forced into a space from where they can offer no threat, that can be a success," he added.
Nearly 2,000 troops from Chad and Niger, with experience of fighting in the Sahara, are backing up the French and Malians as they consolidate their gains in Gao and Timbuktu and also push reinforcements up towards Kidal in the wild northeast.
"The military cycle is far ahead of the political cycle ... They are liberating more than they can occupy," Mann said.
He added Malian authorities also needed to be pragmatic in handling fickle pro-autonomy northern Tuareg rebels, many of them experienced desert fighters, who have offered to help the French-led offensive against al Qaeda and its allies.
Mali's interim President Dioncounda Traore, who says he aims to hold national elections on July 31, said on Thursday he was open to dialogue with the Tuaregs provided they dropped any territorial independence claim.
It was a revolt by the Tuaregs, swelled by arms and fighters from the 2011 conflict in Libya, that initially seized Mali's north following a March military coup in the southern capital Bamako, before being hijacked by Islamist radicals.
But any attempt to placate the Tuaregs could draw a hostile reaction from the Malian public and military, the latter still smarting from its defeat last year by the desert rebels and the massacre of its troops. However, the army's meddling hand in national politics may have been weakened by French intervention.
Mali's army now guards roadblocks and checkpoints behind the French advance, but pockets of rebels still lurk in the bush.
Four Malian soldiers were killed on Wednesday between Gossi and Gao when their vehicle struck a landmine suspected to have been planted by insurgents in a nominally liberated area.
FEARS OF "WILD CARD" STRIKE
The African follow-up force intended to take over security from the French is far from being in place as it grapples with shortages of kit and supplies and lack of airlift capacity.
Besides Chadians and Nigeriens, only around 1,000 other Africans are on the ground in Mali, from Togo, Benin, Nigeria, Senegal and Burkina Faso, out of more than 8,000 soldiers expected to comprise the African force, known as AFISMA.
Advocates of the African force to pacify northern Mali point to the example of the AMISOM African peacekeeping force in Somalia, which now numbers more than 17,000. Deployed in 2007, it has driven al Shabaab militants out of the capital Mogadishu and, more recently, out of the southern port of Kismayu.
But this has been a tough campaign lasting several years, AMISOM has suffered several hundred casualties and countries with troops in Somalia, such as Kenya and Uganda, have experienced militant bomb and guerrilla attacks on their soil.
So even as Malian and French leaders celebrate success on the ground, there is concern Islamist militants inside and outside Mali could strike back, just as they did in the surprise raid on the In Amenas gas plant in Algeria earlier this month.
"The wild card is something completely asymmetrical, like the Algeria gas plant, or an attack in Bamako itself," Mann said, cautioning the war in the Sahara could be long and hard. (Additional reporting by Tiemoko Diallo in Bamako, Richard Lough in Nairobi, John Irish in Paris; Writing by Pascal Fletcher; Editing by Dan Flynn and Jason Webb)