YAOUNDE French special forces have arrived in northern Cameroon to help locate a French family who were kidnapped on Tuesday and moved to Nigeria, a local governor said on Wednesday.
The abduction of three adults and four children highlights the risk to French nationals and interests in Africa since Paris sent forces to Mali to oust Islamist rebels.
"French special forces came in yesterday from N'Djamena to help with the investigation. They left yesterday and came back today," Augustine Fonka Awa, governor of Cameroon's Far North Region, told Reuters by telephone.
He declined to say how many French military arrived from their regional base in Chad's capital, which is about 40 miles (60 km) from where the French tourists were taken.
French Defence Minister Jean-Yves Le Drian said on Wednesday evidence pointed to Nigerian Islamists Boko Haram, but there did not appear to be a direct link to France's intervention in Mali.
"We believe it's the Boko Haram group that carried out the kidnapping, but we don't know for sure. Unfortunately, terror breeds terror," Le Drian told France 2 television.
Asked to confirm or deny whether France had sent special forces, a spokesman at the Defence Ministry in Paris said only that their presence was an unfounded rumour.
"French gendarmes visited the site of the kidnapping yesterday in coordination with Cameroonian police to assess the situation and were protected by French military," he said.
Two Yaounde-based agents from the French DGSE foreign intelligence agency were dispatched to the kidnap zone to work with Cameroon's secret service and a French army helicopter was sent to help look for the hostages, French BFM TV reported without citing sources.
Joseph Dion Ngute, a junior minister at Cameroon's foreign ministry, told French television the kidnappers put the hostages on motorcycles and stole another car before heading to Nigeria.
"Our forces and the Nigerian forces were alerted, but before they reacted the kidnappers had vanished."
Security in the Dabanga area where they were taken, six miles (10 km) from the Nigerian border, has been reinforced and "urgent measures" to locate the family put in place, he said.
It was the first case of foreigners being seized in the mostly Muslim north of Cameroon, a former French colony. But the region - like others in West and North Africa with typically porous borders - is considered within the operational sphere of Boko Haram and fellow Nigerian Islamist militants Ansaru.
The father of the family, which included children aged between 5 and 12, worked for utility GDF Suez and French television reported he was from a family of winemakers in the Burgundy region.
"It is expected that French forces will engage in resolving this issue from within Cameroonian borders, with the support of the Cameroon government," said Nadia Ahidjo of africapractice, an Africa-focused consulting firm.
Nigerian army spokesman Colonel Sagir Musa said "the armed forces were on alert ... ready to apprehend any criminal elements or terrorist that come into our areas."
France has about 6,000 nationals in Cameroon. It issued a travel warning on Tuesday advising its citizens not to travel to the extreme north and for those already there to leave.
Cameroon is a largely secular state where 70 percent are Christian. About 24 percent are Muslim and mainly live in the three northern regions of the country. Until now there have been no known links between with Islamists in northern Nigeria.
Boko Haram poses a big threat to stability in Nigeria, Africa's top oil-producing state. Western governments worry they could link up with other Islamist groups in the region.
France intervened in Mali last month after Islamists seized the north and pushed south towards the capital Bamako.
French-led forces have since driven the Islamists from north Mali towns into remote desert and mountains.
"It's these groups that are calling for the same fundamentalism, whether it's in Mali or in Somalia or in Nigeria. These groups threaten our security," Le Drian said.
French President Francois Hollande said the kidnappings would not stop France from pursuing its operation in Mali.
The kidnapping brought the number of French held hostage in isolated regions of west and north Africa to 15, including one abducted by Nigerian al Qaeda-linked Ansaru in December.