Paris was scrambling Wednesday to verify a claim by Al-Qaeda's north African branch that it has executed a French hostage in retaliation for France's military intervention in Mali.
A man claiming to be a spokesman for Al-Qaeda in the Islamic Maghreb (AQIM) told Mauritania's ANI news agency late Tuesday that "spy" Philippe Verdon had been executed in Mali on March 10 "in response to France's intervention in northern Mali".
"The French President (Francois) Hollande is responsible for the lives of the other French hostages," the spokesman warned.
The French foreign ministry said it was trying to verify the report. "We can't say at the moment if it is credible," said a spokesman.
Verdon's father Jean-Pierre Verdon said he feared the worst. "I'm not under any illusion, but I will wait for confirmation," he told AFP.
In all 15 French nationals, including Verdon, are being held captive in Africa, with AQIM claiming responsibility for six of the kidnappings.
Verdon was seized on the night of November 24, 2011 along with Serge Lazarevic from their hotel in Hombori, northeastern Mali, while they were on a business trip.
The families denied that the two men were mercenaries or spies.
The president of a support committee for the two men, Pascal Lupart, said the French foreign ministry had informed the families early Wednesday about the AQIM statement.
"They told the family to treat it with caution. Nothing is confirmed," Lupart told AFP.
AQIM claimed responsibility for the kidnappings, and in August last year a video showing Verdon describing his "difficult living conditions" was posted on a Mauritanian website.
Extremist groups often use the private news agency ANI to distribute their statements or claims, which often turn out to be accurate.
The French hostages' families have in recent weeks expressed growing fears for their loved ones in the light of France's military offensive aimed at routing Islamists from northern Mali.
"We will do everything to free our hostages," French Prime Minister Jean-Marc Ayrault told parliament on Wednesday.
France has in the past authorised the payment of ransoms to kidnappers in order to secure the release of its nationals, but officials say that has stopped since Hollande came to power last year.
Paris deployed forces in Mali on January 11 to help stop Al-Qaeda linked fighters who had controlled the north of the country since April 2012 from moving southward and threatening the capital Bamako.
France now has more than 4,000 troops on the ground in Mali, of whom about 1,200 are currently deployed in the northeast, carrying out clean-up operations after driving out most of the Islamist rebels from the area.
French troops will begun pulling out of the troubled country "from the end of April", Ayrault told parliament.
Hollande had already said he planned to scale back French military presence in the former colony from next month and start handing over responsibility to Malian troops and an African stabilisation force.
Five French soldiers have died in combat since the start of the operations.
There are still pockets of resistance in some areas such as the main northern city of Gao, which have witnessed stray attacks and suicide bombings since the Islamists fled.
The French troops in the region are backed up by African forces. Soldiers from Chad, whose experience and training has made them key in the French-led offensive, have also suffered casualties with at least 26 deaths.
On Tuesday the French army announced that 15 Islamist fighters had been killed in recent days in the northern Mali region of Gao, with the seizure of a large cache of arms and ammunition.
The AQIM source cited by the Mauritanian news agency refused to confirm reports that top Islamist rebels Mokhtar Belmokhtar and Abdelhamid Abou Zeid had been killed in Mali earlier this month.
France has been carrying out DNA tests to determine whether the militant leaders are among those killed in recent fighting in Mali.
Also on Wednesday, France's foreign ministry spokesman called for the "immediate release" of Malian journalist Boukary Daou, who was arrested this week for "incitement to disobedience" after publishing a letter criticising the new government salary earned by a former junta chief.
Mali's media have said they are boycotting government coverage in protest at the detention.