PARIS: The reported killings of two leading Islamist militants in northern Mali are raising fears over the fates of several French hostages held in the region who may have been used as human shields.

Experts said concerns over the hostages could explain why France has been reluctant to confirm the announcements from Chad of the deaths of prominent extremists Mokhtar Belmokhtar and Abdelhamid Abou Zeid.

Supporters of the hostages and their families said there were real fears they had been caught up in the crossfire during the operations that reportedly led to the Islamists' deaths.

"This is a rumour that is enormously worrying for the families and those of us who support them," said Didier Beguin from the support committee for four French hostages abducted by radical Islamists in Niger in September 2010.

"We can imagine something positive from this and the quick release (of the hostages), but we can also imagine the worst. For now this uncertainty is leaving us even more concerned."

A total of 15 French hostages are being held on African soil, including at least seven being held by Islamist militants in the Sahel region, the semi-arid belt south of the Sahara desert that stretches across a swathe of western Africa.

Chad announced on Saturday the killing of Belmokhtar, the one-eyed Islamist leader who masterminded an assault on an Algerian gas plant in January that left 37 foreign hostages dead.

The country on Friday also announced its troops had killed Abou Zeid, the top commander of Al-Qaeda in the Islamic Maghreb (AQIM) in Mali, during a major battle that also left 26 Chadian soldiers dead.

Both men were directly involved in most of the kidnappings of foreigners that have plagued the region in recent years. Abou Zeid was believed to be holding a number of Western hostages, including the four French citizens kidnapped in Niger.

"Jihadist leaders have a tendency to take hostages with them. This gives them human shields and the ability to quickly launch negotiations if necessary," said Matthieu Guidere, a French university professor and Al-Qaeda specialist.

"If Abou Zeid was killed in a bombing, they could be collateral victims of this attack. If it was in fighting then they would have been hidden not far away. But when the jihadists are attacked, retaliation against the hostages is fairly routine."

France has been in the lead of operations in Mali after its forces launched a lightning intervention in mid-January to oust Islamist rebels who had seized control of the country's vast desert north last year.

It has not confirmed the deaths of either top Islamist and experts said that could be a calculated move aimed at protecting the hostages.

"The lack of a confirmation or denial from France keeps things vague and minimises the incident so as not to feed the desire for retaliation," said Anne Giudicelli, a specialist on Islamic militant groups.

Guidere said the Chadian announcements could even be part of a risky ploy where false information is released in a bid to flush out militant leaders.

"It is quite possible that there is an operation to lie in order to get to the truth. The goal would be to make Abou Zeid emerge to deny his death so his trail can be found again," he said.

Amid so much confusion and doubt, families of the hostages said all they could do was try to stay calm and hope for the best.

"We have to give it some time, we are not sure of anything," said Rene Robert, the grandfather of Pierre Legrand, one of the four hostages held in Niger.

"The rumours are multiplying, we need to keep a cool head and wait."