* Difficult phase sees French soldiers in rebel stronghold
* French national taken prisoner during recent fighting
* Huge weapons cache found in valley area
PARIS, March 8 (Reuters) - French forces searching for al Qaeda-linked militants in the hidden valleys of the remote north of Mali are now deep in the Islamists' sanctuary, France's defence minister said on Friday.
A day after a visit to Mali, Jean-Yves Le Drian said the current phase of the eight-week-old French-led offensive was the hardest, as it required winkling the Islamist fighters out of entrenched positions in the Adrar des Ifoghas mountains.
French soldiers killed about 15 militants this week after discovering a small army of jihadists in the Ametetai valley.
Some 30 French soldiers were wounded in the operation and a French national fighting for the Islamists was taken prisoner, Le Drian told Europe 1 radio on Friday.
"Now it's a bit more difficult because we're in the sanctuary," Le Drian said. "We knew this part of Mali was potentially the sanctuary of AQIM (al Qaeda in the Islamic Maghreb), and we weren't wrong."
"The search is continuing as of yesterday afternoon in the other valleys, because the whole territory has to be cleaned out completely."
The offensive, which began in January, has driven Islamist rebels from most of the swathe of northern Mali they had held since April, but has failed to eradicate pockets of resistance in mountain hideouts and around Gao, the region's largest town.
Le Drian said French forces had retrieved a "very impressive" weapons cache in the Ametetai valley, including heavy arms and material for improvised explosive devices and suicide bomb belts.
"We found them by the ton," Le Drian said. "I hadn't expected this to such an extent."
He said a Frenchman among prisoners taken during combat would soon be extradited to France.
He said French forces still had to clear out northeastern Mali and secure Gao before they can begin to scale back and hand over operations to African troops.
"We are on the right path but until the entire territory has been freed, I remain cautious," Le Drian said. (Reporting By Alexandria Sage; Editing by Alistair Lyon)