By Leigh Thomas and Adama Diarra

PARIS/BAMAKO (Reuters) - One of al Qaeda's senior field commanders in the Sahara, Abdelhamid Abou Zeid, has probably been killed in Mali, the head of France's joint chiefs of staff said on Monday.

Edouard Guillaud's remarks were the first indication from the French government that Abou Zeid, regarded as one of al Qaeda in the Islamic Maghreb's (AQIM) most ruthless operators, died in fighting in the rugged north of Mali.

"It is probable, but only probable. We don't have any certainty for the moment," Guillaud told Europe 1 radio, when asked about Abou Zeid's death. "It would be good news."

Guillaud said the death of the smuggler turned jihadi, who has earned tens of millions of dollars for AQIM with a spate of kidnapping in the Sahara in recent years, could not be confirmed because his body had not been recovered.

Chad's army, which is fighting alongside French troops to clear militants out of northern Mali, said last week it killed Abou Zeid and another al Qaeda commander in the area, Mokhtar Belmokhtar, the mastermind of January's mass hostage taking at the In Amenas gas plant in the Algerian desert.

Guillaud said he was "extremely cautious" about reports of Belmokhtar's death, noting that some militant websites had said the al Qaeda commander, nicknamed 'the uncatchable', was still alive.

Mauritanian news website Sahara Media, which has close links to Belmokhtar and his Mulathameen brigade, cited an Islamist source close to AQIM as saying the one-eyed commander was not even in the Adrar des Ifoghas region, but was operating to the southwest close to the Malian town of Gao.

The unnamed source cited by Sahara Media confirmed Abou Zeid had been killed but said he died in a French air strike not during fighting with Chadian troops.

Abou Zeid is blamed 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.

While his killing would deal a serious blow to al Qaeda's leadership in the region, it also raises questions about the fate of seven French hostages thought to be held in northern Mali.

After a seven-week-old campaign, French, Chadian and Malian troops have pushed Qaeda-linked fighters, who had threatened to take over Mali, back to their mountain and desert hideouts.

France started it ground and air campaign in January, fearing the militants could use their stronghold in the West African nation as a launchpad for international attacks.

Guillaud said French forces had found about 50 supply caches and around 10 workshops for making bombs - explosives which could have been used outside of the immediate region.

"On the ground we are finding literally an industrialization of terrorism," he said.

(Additional reporting by Laurent Prieur in Nouakchott; Editing by Andrew Heavens)