Top Islamist militant leader Abdelhamid Abou Zeid has "likely" been killed in Mali as reported by Chadian forces, the French army's chief of staff, Admiral Edouard Guillaud, said on Monday.
"It is likely, but it is only likely," Guillaud said when asked on Europe 1 radio about the killing, adding that French authorities cannot "have any certainty right now because we have not recovered the body."
On the reported killing of another Islamist rebel leader, Mokhtar Belmokhtar, Guillaud was more sceptical, saying: "I am extremely cautious."
He also said the army did know know where French hostages being held in the Sahel region were currently located but that it believed they were not in areas being hit by French air strikes.
"We do not know where the hostages are," he said. "We think the hostages are not there (where air strikes are taking place), otherwise we would not be carrying them out."
Guillaud said French forces in Mali had uncovered an "industrial terrorist organisation" in the northeast, where French and Chadian troops are hunting down Islamist rebels driven from northern Mali's main cities by a lightning French-led offensive launched in mid-January.
French forces have so far found more than 50 weapons caches, a dozen workshops and 20 improvised explosive devices, he said.
"It shows that this goes beyond... Mali, beyond even the Sahel, it was expansionist," he said.
"We have cleaned out one of the valleys, a main valley where Chadian and French forces moved in about 10 days ago," he said. "We searched all night and from today (Monday) we will move into the other valleys."
Guillaud said the military operation was dealing a fatal blow to Al-Qaeda in the Islamic Maghreb (AQIM), the leading militant group in the region.
"We are breaking the back of Al-Qaeda in the Islamic Maghreb, that was the goal set by the president," he said.
He said the French military was not surprised that fighting had intensified in recent days as Islamist rebels have been cornered in mountain hideouts.
"We knew this would be the hardest part of this campaign," Guillaud said. "We knew that we were dealing with fanatics."
He said he believed there were "several hundred" Islamist rebels remaining in the region at most.
"We estimate that there were between 1,200 and 1,500 fighters in northern Mali when we intervened on January 11 and we think today more than a quarter of them are in the Ifoghas" mountains, he said.