Military helicopters are searching for a French family of seven kidnapped while on holiday in Cameroon, and security around the region is being increased amid tensions over France's role in western Africa.
Cameroonian President Paul Biya ordered tight security measures and urgent steps to free the hostages, who include four children, who were kidnapped by armed gunmen in the country's far north yesterday and whisked toward Nigeria.
A ministry statement said the Cameroonian government is in contact with the Nigerian and French authorities.
Officials suggested the involvement of Boko Haram, one of Nigeria's Islamic extremist sects.
Nigeria's borders were also put on red alert in the hunt for the kidnappers, believed to be in the country or heading to the country, said Nigeria's comptroller general of immigration, Rilwan Musa.
"We have already sent alert messages across the north-east borders and all other borders of the nation," he said. "We have told our men to be on the alert. We have given the border posts all the supports they need to tackle them whether in the day or at night."
The kidnapping came as thousands of French troops are deeply involved in a military intervention against Islamic extremists who had taken control of a big part of the West African country of Mali.
French President Francois Hollande, speaking at a Cabinet meeting, called it an "odious act" and expressed particular horror that children were involved, according to his government spokeswoman.
Speaking in Parliament, French Foreign Minister Laurent Fabius said: "We must do the utmost to free our hostages, but nothing would be worse than giving in."
Meanwhile, in France, spokeswoman Agnes Thibault-Lecuivre, of the Paris prosecutor's office, said it has opened a preliminary investigation into "kidnapping and sequestration by an organised group in relation with a terrorist organisation" following the hostage-taking. France's counter-terrorism agency DCRI is in charge of the probe, she said.