* Death comes as Chad says killed al Qaeda commander Belmokhtar
* French spokesman says cannot confirm Islamist killed
* Militant says Belmokhtar alive, to make statement soon (Adds claim on militant for Belmokhtar is alive)
By Gus Trompiz and Joe Penney
PARIS/GAO, Mali, March 3 (Reuters) - France said on Sunday a third French soldier had been killed in fierce fighting with Islamist rebels in northern Mali but could not confirm Chad's report that its troops had killed the al Qaeda commander behind January's mass hostage-taking in Algeria.
A whirlwind seven-week campaign has driven al Qaeda-linked fighters who took over northern Mali last April into mountain and desert redoubts, where they are being hunted by hundreds of French, Chadian and Malian troops.
France's defence ministry said 26-year-old Corporal Cedric Charenton was shot dead on Saturday during an assault on an Islamist hideout in the desolate Adrar des Ifoghas mountains near Algeria, the third French soldier killed in the campaign.
French army spokesman Colonel Thierry Burkhard said some 15 Islamists were killed in some of the fiercest fighting during the campaign so far but that he could not confirm Chad's claim that its troops had killed al Qaeda commander Mokhtar Belmokhtar at a nearby camp in the remote Ametetai valley.
"We are facing a very fanatical adversary," Burkhard said, noting the Islamists were armed with rocket and grenade-launchers as well as machine guns, AK47 assault rifles and heavy weapons. "They are fighting without giving ground."
The death of Belmokhtar, nicknamed 'the uncatchable', has been reported several times in the past and analysts share caution shown by Paris in confirming his demise.
However, the latest report came a day after Chadian President Idriss Deby said Chadian forces had also killed Abdelhamid Abou Zeid, al Qaeda's other senior field commander in the Sahara.
The killing of Belmokhtar and Abou Zeid, if confirmed, would eliminate al Qaeda's leadership in Mali and raise questions over the fate of seven French hostages thought to be held by the group in northern Mali, an area the size of Texas.
Rudy Attalah, a former senior U.S. counterterrorism official focused on Africa and now head of risk analysis firm White Mountain research, was sceptical about Chad's claim.
He said Belmokhtar had in the past carefully avoided operating in the same area as Abou Zeid and was known as an elusive operator who shifted through the desert in small, mobile groups of fighters.
"I don't think they killed him at all," Atallah said, adding Chad might be seeking to divert domestic attention from its 26 soldiers killed in the operation. "Deby is under a lot of pressure. Announcing these killings redeems his troops."
An unidentified participant in militant website discussions said in a message posted on several jihadi forums that Belmoktar was "alive and well and leading the battles himself", the U.S.-based SITE monitoring service reported on Sunday.
Belmokhtar would soon issue a statement himself, SITE reported the participant saying.
Al Qaeda in the Islamic Maghreb (AQIM) has pledged to avenge the French assault on its fighters in Mali, which Paris said it launched due to fears its former colony could become a launch pad for wider al Qaeda attacks.
Belmokhtar, whose smuggling activities the Sahara earned him the nickname "Mr Marlboro", became one of the world's most wanted jihadis after masterminding the raid on the In Amenas gas plant in Algeria in which more than 60 people were killed, including dozens of foreign hostages.
Abou Zeid is regarded as one of AQIM's most ruthless operators, responsible for the kidnapping of more than 20 Western hostages since 2008. He is believed to have killed British hostage Edwin Dyer in 2009 and 78-year-old Frenchman Michel Germaneau in 2010.
France and Mali have said they could not confirm his death.
French radio RFI and Algerian daily El Khabar have reported that DNA tests were being conducted on members of Abou Zeid's family to confirm whether a body recovered after fighting in Adrar des Ifoghas was indeed the Islamist leader.
Mali's army, meanwhile, said it had killed 52 Islamist rebels in desert fighting some 70 km (45 miles) east of Gao, northern Mali's largest town, with support from French helicopters and ground troops.
"There was a big fight with lots of enemy killed," said Lieutenant Colonel Nema Sagara, the Malian army's deputy commander in Gao. "Our troops went out to battle and they met them. There are no dead on the Malian side." (Additional reporting by Catherine Bremer in Paris, David Lewis in Dakar, William Maclean in Dubai; Writing by Daniel Flynn; Editing by Myra MacDonald)