France has urged its citizens to leave north Cameroon “as quickly as possible” after seven of its nationals were kidnapped by gunmen on Tuesday.
French President Francois Hollande said he suspected militant Islamists from neighbouring Nigeria of being behind the abductions.
He added that the seven tourists, including four children, had probably been taken to northern Nigeria.
The abductions come amid a French-led intervention against Islamists in Mali.
At least eight French nationals are already being held by Islamist groups in Africa.
The French foreign ministry said on its website citizens were “officially advised not to go to the far north of Cameroon (the shores of Lake Chad in the South Maroua), and the border with Nigeria, until further notice”.
Mr Hollande said gunmen from Nigeria’s Boko Haram group may have been behind the kidnapping.
“I see the hand of Boko Haram in that part of Cameroon, and that is worrying enough for us to mobilise,” Mr Hollande said.
Boko Haram has not yet commented on the allegation.
Mr Hollande said everything possible was being done to rescue the seven, all from the same family.
They had been returning from a visit to Waza National Park when they were attacked by men on motorcycles, Cameroonian officials said.
Boko Haram has staged many attacks across northern Nigeria in recent years, targeting churches, government buildings and the security forces.
“If everything is confirmed, this signifies that the fight against terrorist groups is a necessity,” said French Foreign Minister Laurent Fabius, quoted by the Associated Press news agency.
“There is a battle to be led by the international community against terrorist groups and narco-terrorists.”
Another Islamist group, Ansaru, is also active in northern Nigeria.
It says it abducted seven foreign workers in northern Nigeria on Sunday to avenge “transgressions” by Western forces in Mali and and Afghanistan.
Italian, British, Greek and Lebanese workers are thought to be among those held after an attack on a construction project in Bauchi state.
Ansaru also says it is holding a French national, Francis Colump, who was seized in the northern state of Katsina last year.