* Militants hoped for "terrorist sanctuary"-French minister
* French forces attack rebels holed up in mountains
* Huge weapons cache found in isolated valley
* French national fighting for Islamists is captured (Recasts with fresh Le Drian quotes, adds details, changes dateline from Paris)
By Tiemoko Diallo
BAMAKO, March 8 (Reuters) - French forces in Mali have discovered tonnes of weapons stockpiled by al Qaeda-linked fighters who planned to use the country's north as a base for international attacks, France's defence minister said on Friday.
France launched a ground and air operation on Jan. 11 to break Islamist rebels' hold on the northern two-thirds of Mali, saying the militants posed a risk to the security of West Africa and Europe.
While the rapid offensive has now taken back most of the territory seized by the militants nearly a year ago, French and allied Chadian forces have met heavy resistance from militants holed up in mountains near the Algerian border.
Speaking at the end of a two-day visit to Mali on Friday, French Defence Minister Jean-Yves Le Drian said the Islamist rebels, many of whom have flooded in from abroad, had been well armed and hoped to make the impoverished, arid nation a "terrorist sanctuary".
"What struck me most was the scale of the arsenals that we have discovered in the north and in the region of Gao. There was certainly the desire to make it a base for international actions," he told journalists in the capital Bamako.
French soldiers this week killed about 15 militants and captured a French national fighting for the Islamists, after discovering a small army of jihadists in the isolated Ametetai valley. About 30 French soldiers were wounded in the clashes.
Earlier on Friday, Le Drian told Europe 1 radio that French forces had found rebel weapons "by the tonne" and that one cache uncovered in the Ametetai valley included heavy arms, material for improvised explosive devices and suicide bomb belts.
He said the Frenchman who was seized, among a number of prisoners taken in combat, would be extradited to France soon.
MORE FIGHTING TO COME
France intervened in its former colony after the Malian government requested help against the rebels.
French President Francois Hollande said on Wednesday his country would start to reduce its forces in Mali from April, a month later than previously forecast, with the military exit taking place over several months.
Le Drian said French forces still had to drive militants from Mali's northeastern border area and secure the rebels' former eastern stronghold of Gao before they could scale back.
"We are 70 percent there ... but we must do 100 percent," he said on Friday. "The mission given to our forces by the President of the Republic is to succeed in liberating all Malian territory ... so there will be more combat."
Chad claimed to have killed al Qaeda's two top leaders in the region, Abdelhamid Abou Zeid and Mokhtar Belmokhtar, last week. Hollande said on Wednesday that "terrorist leaders" had been killed in the operation, but did not elaborate.
Despite France's military successes to date in Mali, the planned withdrawal of its troops could yet face obstacles.
Islamist rebels have struck back with bombings and raids in areas considered liberated by French and African forces, raising concerns that Paris could become mired in a messy guerrilla war.
A French soldier patrolling with Malian troops was killed on Wednesday near Gao, which was retaken from rebels in late January.
Four civilians were killed in an ambush on a vehicle late on Thursday outside the town of Tonka, about 100 km (65 miles) southwest of the ancient northern city of Timbuktu, though it was unclear whether the attackers were rebels.
"They were shot in the head and chest. Everything in the vehicle was taken," Papou Hidara, the son of one of the victims, told Reuters.
An 8,000-strong African force is supposed to replace the French military presence in the country but it has struggled with logistical and financing setbacks and is currently deployed mainly in the south.
It is seeking a U.N. peackeeping mandate, which would guarantee a budget for its operations, but a decision from the U.N. Security Council is still weeks, if not months, away. (Additional reporting by Alexandria Sage in Paris, Rainer Schwenzfeier and Adama Diarra in Bamako; Writing by Joe Bavier; Editing by Pravin Char)