PARIS/BAMAKO: France gave the first indication on Monday that it believed reports that Abdelhamid Abou Zeid, one of al Qaeda's most feared commanders in Africa, had been killed in Mali, an event that would deal a serious blow to the militants' leadership.

Edouard Guillaud, head of France's joint chiefs of staff, said it was probable Abou Zeid had been killed in the Adrar des Ifoghas mountains where French troops are hunting down al Qaeda-linked fighters after a seven-week campaign which has broken Islamist control of northern Mali.

But he remained cautious over reports of the death of another jihadist leader, Mokhtar Belmokhtar.

Abou Zeid is accused of earning al Qaeda millions of dollars through kidnappings, including the abduction 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.

Guillaud said his death could not be definitively confirmed because his body had not been recovered.

"It is probable, but only probable," he told Europe 1 radio. "We don't have any certainty for the moment. It would be good news."

Chad, which is fighting alongside France in the mountains, said last week its troops had killed both Abou Zeid and Belmokhtar, the mastermind of a mass hostage-taking in January at the In Amenas gas plant in the Algerian desert in which around 60 people were killed.

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 at large.

If confirmed, the killing of Abou Zeid and Belmokhtar would raise questions about the fate of eight French hostages held by al Qaeda in the Sahel. The families of four French hostages seized in Niger in September 2010 appealed to Paris on Monday to open negotiations with al Qaeda in the Islamic Maghreb (AQIM).

"We ask for a pause to negotiate with the remaining fighters and save our loved ones, rather than fighting on blindly and taking the risk of putting their lives in danger," Pascale Robert, the mother of one of the hostages, told BFM TV.

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

Although it has secured logistical and intelligence help from other nations, French troops are the only Western forces involved in combat operations.

British Foreign Secretary William Hague briefly visited Mali on Monday. He reiterated London's support for the mission but said it would be limited to transport and training for African forces. "We will not send troops to northern Mali," he said.

Mauritanian news website Sahara Media, which has close contacts with Abou Zeid's wing of AQIM, cited an Islamist source as confirming the smuggler turned jihadist had been killed in a French air strike.

But the website said the unnamed Islamist source denied reports of Belmokhtar's death, 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 general commanding Chadian forces in northern Mali, Oumar Bikomo, struck a more cautious tone than the government in N'Djamena over Belmokhtar's fate.

"It's possible that he has been killed," he told journalists during a visit to northern Mali. "But as there have been no DNA tests carried out one can still have some doubts."

Chadian soldiers showed a Reuters reporter a photograph of a dead Islamist fighter who they said could be Belmokhtar but the bearded man's face had been badly wounded in fighting and it was impossible to identify him.

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.

The French army has tightly restricted journalists' access to the region but on Monday released footage showing French soldiers sorting through a vast cache of arms - including a howitzer, military trucks, hundreds of artillery shells - seized in the Ikoufi valley.

"On the ground we are finding literally an industrialisation of terrorism," Guillaud said.