President Francois Hollande vowed Tuesday not to give in to jihadists who have threatened to kill a French hostage in Algeria, as troops raced against time to find him.
Algerian military planes were combing the mountainous eastern Tizi Ouzou region backing elite anti-terrorist units on the ground in a desperate bid to find 55-year-old Herve Gourdel, a security source said.
The kidnapping was claimed by Jund al-Khilifa (Soldiers of the Caliphate) -- a group linked to Islamic State jihadists -- that has vowed to kill the hostage by Tuesday night if Paris did not stop air strikes in Iraq.
As the deadline loomed, some 20 truckloads of paratroopers also joined the search, a witness said, and police set up roadblocks along the highway that snakes through the mountains.
"As grave as this situation is, we will not give into any blackmail, any pressure, any ultimatum, no matter how odious, how despicable," Hollande said in New York, where he is due to attend the annual UN General Assembly.
France opened an official inquiry into the kidnapping, which took place on Sunday in the heart of Algeria's Djurdjura National Park, whose dense forests, deep gorges and picturesque lakes were once a major draw for tourists.
However, the mountains became a sanctuary for Islamists in the 1990s who would later swear allegiance to Al-Qaeda, and security forces have been unable to dislodge them.
Jund al-Khilifa was formed recently after splintering from Al-Qaeda in the Islamic Maghreb (AQIM), which experts say has raked in some $120 million (93 million euros) in ransom payments in the previous eight years.
Paris -- which has denied accusations it pays ransoms -- refused to be cowed by the threat against Gourdel, and Hollande insisted the country would press on with air strikes alongside the United States.
Earlier Tuesday, French Prime Minister Manuel Valls had warned that if Paris gave in, it would "be handing them victory".
The French government confirmed as authentic a video posted by the group showing the white-haired and bespectacled hostage squatting on the ground flanked by two hooded men clutching Kalashnikov assault rifles.
The video surfaced shortly after IS issued a statement urging Muslims to kill Westerners whose nations have joined a campaign to battle the jihadist group, in particular Americans and French.
The United States has built a broad coalition of more than 50 nations to fight the IS organisation, after the jihadists seized large parts of Syria and Iraq and committed widespread atrocities, including beheadings and crucifixions.
On Friday, France conducted its first air strikes in Iraq.
"These strikes, this engagement will of course continue," promised Valls.
The country "cannot be scared of those who threaten it in this way", he said, adding: "I want to convince our compatriots ... that we have never been up against such a threat, in France and in Europe."
However, while the United States launched strikes against the Islamic State jihadists in Syria, France has said it would limit its strikes to Iraq.
In a YouTube video posted on Monday night, Jund al-Khilifa, which has pledged allegiance to IS, threatened to kill Gourdel within 24 hours unless Paris halts its air strikes.
Gourdel lives in the southern French city of Nice and works as a mountain guide. He arrived in Algeria on Saturday and was seized a day later while hiking with Algerian friends.
The 55-year-old is a passionate photographer who has travelled widely to countries such as Morocco, Jordan and Nepal.
"I have always wanted to capture these incredible landscapes. It was in the Moroccan Atlas mountains that I began to progress. I wanted to bring back images of the people who lived there," he explains on his website.
Gourdel's family expressed hope for his freedom and spoke of their anguish.
"We learned with great pain of the kidnapping of Herve Gourdel, our father, our partner, our son, our brother and our very dear friend," said a family statement sent to AFP.
"Herve, we can't wait to see you again and we are waiting for you," the statement added.
His 82-year-old mother told the website of L'Express weekly that the kidnapping was "an ordeal for us".
She last heard from her son on Sunday.
"Everything was fine, he said he was going for a two-day hike and that it would be hard to reach him," she said.