Kidnappers in Cameroon who seized a French family of seven have fled with them across the border into Nigeria.
Cameroonian President Paul Biya ordered tight security measures and urgent steps to free the hostages.
His comments came as a local governor said French special forces had arrived in northern Cameroon from their regional base in Chad to help an investigation to locate the family.
"French special forces came in yesterday from N'Djamena to help with the investigation. They left yesterday and came back today," said Augustine Fonka Awa.
French army spokesman Thierry Burkhard declined to comment.
The original kidnapping took place on Tuesday at Sabongari, seven kilometres from the northern village of Dabanga near the Nigerian border.
Energy firm GDF Suez confirmed that one of its workers had been kidnapped with his family.
French President Francois Hollande said during a visit to Athens that he was informed of the kidnapping by a Nigerian "terrorist group that we know well", without naming it directly.
France had done everything possible to prevent the kidnappers moving their victims to Nigeria, he said.
GDF Suez said the employee was based in Yaounde, Cameroon's capital, and that the family were holidaying in the north of the west African country.
A Western diplomat in the region said six armed kidnappers on three motorbikes had abducted a couple, their four children and an uncle. The children are aged five, eight, 10 and 12, the diplomat said. A source close to the French embassy in Yaoundé said the family had earlier visited Waza National Park in northern Cameroon.
Asked whether the kidnapping could be a reprisal for France's military offensive against al-Qaeda-linked groups in northern Mali, Hollande spoke of Nigeria's Islamist Boko Haram group.
"I note in particular the presence of a terrorist group, namely Boko Haram, in that part of Cameroon, and that's worrying enough," he said.
French Foreign Minister Laurent Fabius was more explicit, saying the location of the attack, near the border and, further away, the Nigerian city of Maiduguri, where Boko Haram was founded, was "why we think it might be this sect".
Some Boko Haram members are believed to have trained with militants of al-Qaeda in the Islamic Maghreb (AQIM) in northern Mali.
Boko Haram's campaign of violence against the Nigerian security forces - by some estimates claiming 800 lives last year - had not previously included kidnappings. But they have increasingly become a regular tactic of a breakaway group, Ansaru, which has claimed responsibility for several recent abductions.
The group issued a statement linking an earlier kidnapping to the French intervention in Mali.
Meanwhile, Nigeria was the scene of yet another kidnapping, with armed pirates who stormed an oil-service ship off the country's coast saying yesterday they had abducted six foreigners, and demanding a US$1.3 million ransom for their release.
"Three of those abducted are from Ukraine, two from India, one from Russia," Bayelsa state police spokesman Fidelis Odunna said of the attack on Sunday.
Additional reporting by the New York Times