A French national was kidnapped in eastern Algeria on Sunday, France's foreign ministry said, and his kidnappers issued a video threatening to kill him if Paris did not halt its intervention in Iraq.
The Caliphate Soldiers, a group linked to Islamic State militants, published a video on the Internet soon after the French ministry's announcement on Monday, claiming responsibility for the kidnapping and showing a man who identified himself as Herve Gourdel, 55, from Nice in southern France.
The group said it would kill Gourdel if Paris did not halt its intervention in Iraq.
The French foreign ministry later confirmed the video was authentic.
In New York, French Foreign Minister Laurent Fabius said the taking of a French hostage would not deter French participation in a U.S.-led coalition of nations against Islamist State militants.
"We will do everything we can to liberate hostages," Fabius, in New York for U.N. General Assembly, told reporters. "But a terrorist group cannot change France's position."
The kidnapping came just hours after Islamic State spokesman Abu Muhammad al-Adnani urged the group's followers to attack citizens of the United States, France and other countries which have joined a coalition to destroy the radical group.
"A French national was kidnapped on Sunday in Algeria, in the region of Tizi Ouzou, while he was on holiday there," deputy Foreign Ministry spokesman Alexandre Georgini said in a statement.
Citing an interior ministry statement, Algeria's state news agency APS said the Frenchman, who it described as a mountain guide, had been taken in the village of Ait Ouabane when he was traveling in a vehicle with some Algerian nationals.
France, which on Monday raised the threat level at 30 of its embassies across the Middle East and Africa, launched its first air strikes targeting Islamic State targets in Iraq on Friday. It has said all must be done to rid the region of the group.
President Francois Hollande said in a statement he had spoken to Algerian Prime Minister Abdelmalek Sellal and that the two countries were cooperating at all levels to find and liberate the hostage.
Western diplomats and intelligence sources say they believe there are less than 10 hostages still held by Islamic State. The group has recently killed two Americans, James Foley and Steven Sotloff, and one Briton, David Haines, and threatened to kill another Briton, Alan Henning.
The kidnapping was one of the first abductions of a foreigner by militants in Algeria since the North African country ended its decade-long war with Islamist fighters in the 1990s.
The area where the Frenchman was taken is a mountainous region which was once a stronghold for the fighters. There have been several kidnappings targeting Algerian businessmen for extortion in the area but most were freed by security forces.
Al Qaeda's North Africa branch, AQIM, and other groups are still active in Algeria.
Caliphate Soldiers of Algeria
The four-minute video that appeared on YouTube and Islamic State Twitter accounts on Monday was entitled "A message from the Caliphate Soldiers in Algeria to the dog Hollande."
The Caliphate Soldiers said in a statement on Sept. 14 it had split from AQIM and sworn loyalty to the Islamic State.
The video opens with images of Abu Bakr al-Baghdadi, commander of the Islamic State, while in the background Monday's speech from Islamic State spokesman Adnani is played threatening France, coalition allies and Iran.
"We, the Caliphate Soldiers in Algeria, in compliance with the order of our leader Caliph Abu Bakr al-Baghdadi ... give Hollande, president of the criminal French state, 24 hours to cease its hostility against the Islamic State, otherwise the fate of his citizen will be slaughter.
"To save his life, you must officially announce the end of your hostility against the Islamic State," a speaker on the video said.
The video then shows Gourdel sitting next to two armed gunmen in black turbans and carrying assault rifles. He said he arrived in Algeria on Sept. 20 and was taken on Sept. 21.
"I am in the hands of Jund al-Khilifa (Caliphate Soldiers), an Algerian armed group. This armed group is asking me to ask you (President Hollande) to not intervene in Iraq. They are holding me as a hostage and I ask you Mr. President to do everything to get me out of this bad situation. I thank you."
Gourdel's friends and relatives confirmed to a Reuters reporter in Nice that the man in the video was Gourdel.
Local private Echorouk television and agency APS said Algerian military had began a search operation in the area.
Militant attacks and operations are rarer now in Algeria. But at the start of 2013, Islamist militants attacked the Amenas gas plant in southern Algeria, triggering a siege during which 40 oil workers, mostly foreigners, were killed.