The leader of al-Qaeda's offshoot in North Africa says the group has killed a French engineer taken hostage in Niger in April.

In an audio message broadcast Sunday on Al-Jazeera, Abdelmalek Droukdel said the group killed Michel Germaneau in retaliation for the killing of six al-Qaeda members in a recent raid by Mauritanian forces aided by the French military.

Germaneau, 78, was abducted April 22 in Niger and officials believed he was subsequently taken to Mali. Al-Qaeda demanded in several internet messages addressed to French President Nicolas Sarkozy that France help negotiate the release of the group's prisoners in countries in the region.

"Sarkozy has [not only] failed to free his compatriot in this failed operation, but he opened the doors of hell for himself and his people," Droukdel said.

"As a quick response to the despicable French act, we confirm that we have killed hostage Germaneau in revenge for our six brothers who were killed in the treacherous operation," he said.

French government officials would not immediately comment on Sunday's message.

The precise circumstances of the recent military raid in northwest Africa remain a mystery. In announcing the operation on Friday, the French Defence Ministry would not say when or where the raid took place.

The Spanish newspaper El Pais and other media have reported that the raid took place early Thursday and was an attempt to free the French hostage. But El Pais said the troops did not find Germaneau, who had worked for Algeria's oil industry.

The offshoot of the terror network, known as "al-Qaeda in the Islamic Maghreb," had given France until Monday to help secure the release of its jailed members in the region, warning that the aid worker would be executed if France failed to comply.

The group is also holding two Spanish aid workers, Roque Pascual and Albert Vilalta, who were taken hostage in Mauritania in November.