Heavy fighting in northern Mali has left at least 50 rebel fighters and a French soldier dead, officials said Sunday, as Chad promised to prove its soldiers had killed two top Islamist militants.

French Defence Minister Jean-Yves Le Drian called for "caution" on Chad's reports that its soldiers in Mali recently killed Mokhtar Belmokhtar, the mastermind of January's assault on an Algerian gas plant that left 38 hostages dead, and Al-Qaeda in the Islamic Maghreb (AQIM) commander Abdelhamid Abou Zeid.

Le Drian also said a French soldier had died "in some of the heaviest fighting that we have carried out on Malian territory". Both French and Malian officials said clashes had intensified in the region in recent days.

As foreign governments sought confirmation of the reported killing of Belmokhtar and Abou Zeid, Al-Qaeda's two historic leaders in Africa's Sahel region, Chadian Communications Minister Hassan Sylla said his government was "not just wasting breath" with the announcements.

"We are soon going to present some prisoners, some lieutenants who were with them," he told AFP.

But France's Le Drian warned: "A rumour that's repeated over and over doesn't become a fact.

"I call for caution and a spirit of responsibility with regards to information that we are not in a position to materially confirm at this stage," he said in comments released by the defence ministry.

On the ground, a Malian military source said the fighting continued on Sunday near the northern town of Gao, where Malian troops backed by French forces are hunting down Islamist rebels driven from the region's main cities after a lightning French intervention launched on January 11.

"At least 50 MUJAO Islamists 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, an Al-Qaeda offshoot.

France said 26-year-old Corporal Cedric Charenton had been killed on Saturday evening in northern Mali, the third death of a French serviceman since the intervention began.

The French army said at least 15 Islamist rebels had been "neutralised" -- killed or wounded -- in the fighting Saturday that led to the soldier's death.

-- 'Blow to terrorism'? --

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 will have 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."

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.

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

The spectacular assault on the isolated facility, which was jointly operated by British, US and Norwegian oil companies, finished in a bloodbath 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 Belmokhtar, whom a former French intelligence chief had 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.

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