* Video shows adults, children surrounded by gunmen
* Kidnappers seek release of Islamists in Cameroon, Nigeria
* Video follows confusion over hostages' whereabouts (Adds analyst, background, detail)
By John Irish and Bate Felix
DAKAR, Feb 25 Gunmen claiming to be from Nigerian Islamist group Boko Haram 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 in an attempt to establish 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.
"They want the liberation of their brothers in Cameroon and their women imprisoned in Nigeria," the man added, reading notes in French as he sat on a red rug on the floor.
Beside him sat a woman dressed in a black veiled dress, another man and four young children.
The hostage-taking highlighted the risk to French citizens in Africa after Paris sent thousands of troops into Mali last month to oust Islamists operating in the country's vast desert north.
"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 kidnapping brought to 15 the number of French citizens being held in the region.
"WAR ON ISLAM"
France has carried out hundreds of air strikes and dispatched 4,000 troops to Mali to defeat the mixture of al Qaeda-linked groups that hijacked a separatist Tuareg rebellion and occupied the northern two-thirds of the country.
After swift victories in Mali's main towns, French troops risk becoming bogged down in a bloody conflict against an enemy that is using guerrilla tactics and suicide bombs, and has pledged reprisal attacks across the region.
"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," he added, with a pistol at his feet.
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.
The Nigerian militant group has previously posted videos in Hausa, a language spoken in northern Nigeria. Yet the video, whose date of recording was not clear, was only in Arabic.
The black and white flag that hung behind the hostages in the released video is more associated with groups tied to al Qaeda than Boko Haram.
A spokesman for Boko Haram had denied any connection with the kidnapping at the weekend, saying it remained committed to a ceasefire.
However, security experts in Nigeria, Africa's largest oil exporter, say Boko Haram is made up of multiple cells, without a defined command structure.
"It is therefore not unsurprising that you have one group claiming a ceasefire, just as another splinter cell is raising its profile with this kidnapping," said a Nigeria-based Western security source, asking not to be named.
The militant group is known to have had some links to al Qaeda factions in North Africa and Mali, where fighters are believed to have spent some time training.
Although limited for now, the conflict in Mali appears to have deepened connections between mainly Arab Islamist militants in North Africa and black African movements south of the Sahara. (Additional reporting by Asma Alsharif in Cairo, Joe Brock in Abuja and Elizabeth Pineau in Paris; Writing by David Lewis; Editing by Tom Pfeiffer)