BAMAKO  - Heavy clashes in northern Mali have left at least 50 rebel fighters and a French soldier dead, officials said Sunday, following reports that two top militants were killed in recent days.

The reports from Chad that its soldiers killed Mokhtar Belmokhtar, the mastermind of January’s assault on an Algerian gas plant that killed 37 foreign hostages, as well as Al-Qaeda in the Islamic Maghreb (AQIM) commander Abdelhamid Abou Zeid have not been confirmed by other sources. But both French and Malian officials said clashes had intensified in the region in recent days, with French Defence Minister Jean-Yves Le Drian saying a French soldier had died “in some of the heaviest fighting that we have carried out on Malian territory.”

A Malian military source said the fighting continued on Sunday near the town of Gao in northern Mali, where Malian troops backed by French forces are hunting down rebels driven from the region’s main cities after a lightning French intervention launched in mid-January. “At least 50 MUJAO rebels have been killed since the day before yesterday (Friday),” the source told AFP, referring to rebels from the Movement for Oneness and Jihad in West Africa.

France said one its soldiers had been killed on Saturday evening in north Mali, the third death of a French serviceman since the intervention began. The defence ministry identified the soldier as 26-year-old Corporal Cedric Charenton, from the First Parachute Chasseur Regiment, who had been deployed in Mali since January 25 and had previously served in Afghanistan and Gabon.

France has so far suffered relatively few casualties during its operations in Mali. A legionnaire with the 2nd Foreign Parachute Regiment was killed in clashes on February 19 and a helicopter pilot died at the very start of the operation. The French army said at least 15 rebels had been “neutralised” - killed or wounded - in the fighting Saturday that led to the soldier’s death.

The increase in clashes came as Chad announced the killings of Belmokhtar on Saturday and Abou Zeid on Friday. If the killings are confirmed, the French-led military coalition fighting in northern Mali will have eliminated the Sahel region’s two historical Al-Qaeda leaders and decapitated the country’s Islamist insurgency.

“It would be a blow to terrorism and to the criminal network around this man and other people,” British Foreign Secretary William Hague told BBC television in reaction to reports of Belmokhtar’s death.

“These are reports from Chadian soldiers who have been doing a lot of the fighting in northern Mali. We can’t absolutely confirm this at the moment - I stress that,” he said.

Chad’s communications minister, Hassan Sylla, Sunday defended his country’s claims. “We are going to present soon some prisoners, some lieutenants who were with them (the chiefs),” he told AFP.

The Chadian army said Belmokhtar, an Algerian national who had broken away from AQIM weeks ago to form a group called Signatories in Blood, was killed during an operation in Mali’s Ifoghas mountains.

In January, days after France’s surprise decision to send in fighter jets and troops to help the Malian government reconquer the north, Belmokhtar claimed the attack on the In Amenas gas plant in southern Algeria.

The spectacular assault on the isolated facility, which was jointly operated by British, US and Norwegian oil companies, ended in a bloodbath, with 38 hostages killed by the time an Algerian raid ended the crisis. Among the victims were 37 foreigners, from nations including Britain, Norway and Japan.

The report of the death of the man branded “The Uncatchable” came after Chadian President Idriss Deby Itno announced Friday that his forces had killed Abou Zeid, the top AQIM commander in Mali. Deby said his troops killed Abou Zeid during a major battle on February 22 that also left 26 Chadian soldiers dead, but some local media reported he was killed in a French air strike.

Abou Zeid, 46, was believed to be holding a number of Western hostages, including four French citizens kidnapped in Niger in 2010. He and Belmokhtar were directly involved in most of the kidnappings of foreigners that have plagued the region in recent years. The reports of their killings have raised fears over the fates of several French hostages held in the region who may have been used as human shields.