Al Qaeda's North African wing claimed responsibility on Friday for the kidnapping of Frenchman Michel Germaneau who was seized in northern Niger in April, the U.S.-based SITE Intelligence Group said.
SITE, which monitors Islamist websites, said al Qaeda in the Islamic Maghreb (AQIM) had posted an audio message from 78-year-old Germaneau, his picture and a copy of his identification card.
The militants demanded a prisoner exchange and said French President Nicolas Sarkozy would be responsible for the life of the captive, SITE said in its report.
AQIM said Germaneau, who was seized in the Sahara desert region on April 22, was a retired engineer who had worked in the Algerian oil sector.
Military sources in Niger have said an Algerian driver seized with Germaneau had been freed and found wandering through the desert in Mali in late April.
Governments have little influence in the desert region where bandits, smugglers, former rebels and groups linked to al Qaeda operate.
The militants in the Sahara have so far not been able to stage any large-scale attacks, but Western diplomats say cash they are accumulating from a series of kidnappings of foreigners will make them a more potent threat.
Ransoms are usually demanded by the militants in exchange for freeing the hostages.
Western countries say if action is not taken, al Qaeda militants could turn the Sahara into a safe haven along the lines of Somalia or Yemen and use it to launch attacks.
Mali, Niger, Algeria and Mauritania have launched a joint military headquarters in southern Algeria which will triple the number of troops at its disposal to 75,000 within two years.
(Editing by Ralph Gowling)