* French forces clash with rebels in north Mali mountains
* Hollande says last phase is to catch rebel leaders
* French army finds Russian-made rocket launchers (Adds quotes, background, weapons find)
By Vicky Buffery and John Irish
PARIS/DAKAR, Feb 19 (Reuters) - A French soldier and more than 20 Islamist rebels were killed during what appeared to be the first clashes in the Adrar des Ifoghas mountain range where militants have taken refuge in northern Mali, French officials said on Tuesday.
Speaking on a visit to Athens, French President Francois Hollande said serious fighting had broken out and was continuing in the remote area that straddles the Mali-Algeria border, resulting in several casualties among the rebels and one French legionnaire.
"At this moment we have special forces that are in an extremely precarious zone of the Ifoghas," Hollande said. "It's where the terrorist groups that we stopped before have pulled back to."
The soldier is the second French casualty since Paris intervened in Mali last month when Islamist rebels, after hijacking a rebellion by ethnic Tuareg MNLA separatists to seize control of the north in the confusion following a military coup, pushed south towards the capital Bamako.
Highlighting the risk of attacks on French nationals and interests in Africa since the intervention in its former colony, a French family of seven was kidnapped in northern Cameroon on Tuesday by suspected Nigerian militants.
After driving the bulk of the insurgents from northern towns such as Timbuktu and Gao, France has been focusing its operations on Mali's remote northeast mountains, where French special forces and Chadian troops are hunting rebel bases.
They believe the rebels are holding some of the eight French hostages, previously seized in region, in hideouts in the Adrar des Ifoghas range.
"We are now in the last phase of the operation," Hollande said. "That means arresting the last leaders of these groups that are in the extreme north of Mali."
The French defence ministry said that a parachute regiment of 150 soldiers supported by a heavy vehicle patrol and Mirage fighter jets had come under fire on Tuesday morning.
The French raid was aimed at disrupting the militants and dismantling their camps, the ministry said.
"The French troops was able to locate terrorist elements in their hideout, to chase them and to kill more than 20 of them," it said.
ROCKET LAUNCHERS FOUND
French leaders have said they intend to start pulling out the 4,000 French troops in Mali in March to hand over security to the Malian army and to the U.N.-backed AFISMA force, which is expected to exceed 8,000 soldiers and is drawn mainly from Mali's West African neighbours.
French and Malian troops secured the north Mali town of Bourem on Sunday, tightening their control over areas where Islamist insurgents have been launching guerrilla attacks to harass the French-led military operation.
But showing just how well-armed the insurgents are, the French defence ministry said earlier on Tuesday it had found three abandoned Russian-made rocket launchers left behind by Islamists near Bourem.
The BM-21 launch vehicles add to a collection of rockets, boxes of ammunition and accessories previously found in other towns and in all likelihood seized from Libya after the fall of Muammar Gaddafi and after Malian forces retreated last year.
"You have the full spectrum," James Bevan, head of Conflict Armament Research, a group that identifies and tracks weapons, told Reuters after viewing photos of an abandoned cache in Diabaly earlier this month.
"This is pretty heavy ordnance - a level that would achieve parity with or even out gun most West African militaries." (Additional reporting by Bate Felix in Dakar, Jean-Baptiste Vey in Athens and Alexandria Sage in Paris; Writing by John Irish; Editing by Michael Roddy)