By John Irish and Bate Felix
DAKAR (Reuters) - Gunmen claiming to be from Nigeria's Boko Haram Islamist group threatened on Monday to kill a kidnapped French family of seven if authorities in Nigeria and Cameroon do not release Muslim militants held there.
French ministers said they believed the three adults and four children seized in Cameroon's far north near the Nigerian border on Tuesday were being held by Boko Haram which has killed hundreds to try to carve out an Islamist state in Nigeria.
The first sign of the family since they were captured came in a video posted on YouTube in which they appeared surrounded by three gunmen wearing turbans and dressed in camouflage.
"We have been taken by Jama'atu Ahlis Sunna Lidda'awati wal-Jihad," one of the male hostages said in the video, referring to the name in Arabic of Nigeria's Boko Haram militants.
"They want the liberation of their brothers in Cameroon and their women imprisoned in Nigeria," the man added, speaking in French as he sat on the floor beside another man, a veiled woman and four children.
"A video of the French family kidnapped in northern Cameroon last Tuesday has just been posted by Boko Haram," said French Foreign Minister Laurent Fabius. "These images are terribly shocking and show a cruelty without limits."
The hostage-taking highlighted the risk to French citizens in Africa after Paris sent thousands of troops into Mali last month to oust al Qaeda-linked Islamists operating in the country's vast desert north.
"The president of France has launched a war on Islam and we are fighting it everywhere," said one of the apparent kidnappers, speaking in Arabic and identifying himself as a member of Boko Haram. "Implement our demands. If you leave out even one, we will kill these people."
Boko Haram, a Nigerian militant group, has previously posted videos in Hausa, a language spoken in northern Nigeria. The black and white flag that hung behind the hostages in the released video is more associated with al Qaeda-linked groups.
A spokesman for Boko Haram had denied any connection with the kidnapping at the weekend.
However, security experts in Nigeria, Africa's largest oil exporter, say that Boko Haram is made up of multiple cells, without a defined command structure.
The militant group is known to have had some links to al Qaeda factions in North Africa and Mali, but experts say they appear limited for now.
Cameroon's Communication Minister Issa Tchiroma Bakary said he could not comment because his government was not aware of the video.
The governor of Cameroon's Far North Region, Augustine Fonka Awa, said he was not aware of any Boko Haram members being held in the country.
(Additional reporting by Asma Alsharif in Cairo and Joe Brock in Abuja; Writing by David Lewis; Editing by Daniel Flynn and Jon Hemming)