Pressure is growing for Western military drones to be used in the hunt for more than 200 schoolgirls abducted in Nigeria, as the terrorist thought to be holding them threatens to "marry off" girls aged as young as 9.
Experts said unmanned spy planes of the kind used in Afghanistanand satellite-operated cameras could be used to search the vast areas of desert and woodland where the girls are thought to be hidden.
Nigerian military personnel were massing for a search in the Sambisa Forest region. According to Nigerian police, 223 girls are missing after being abducted from their boarding school in Chibok on April 14.
The urgency of the situation was underlined when Boko Haram, the Islamist extremist group believed to have abducted the girls, issued a video in which its leader gloatingly threatened to sell them as "slaves".
"I abducted your girls," said Abubakar Shekau, standing in front of an armoured personnel carrier. Six masked gunmen flanked him. "I will sell them in the market, by Allah. I will marry off a woman at the age of 12. I will marry off a girl at the age of 9."
Shekau described the abduction as part of Boko Haram's war against Western-style secular education in Nigeria, in which the group has attacked dozens of schools in the predominantly Muslim north. "I said Western education should end," he added. "Girls, you should go and get married."
US State Department spokeswoman Marie Harf said yesterday there was concern that many of the schoolgirls had been moved out of the country, possibly to neighbouring Chad or Cameroon.
President Goodluck Jonathan said he had asked the United States, Britain, France and China for help on security issues. He did not specify what help he had sought, but it is thought he may have discussed whether satellite or drone technology could be used.
One satellite technology expertsaid: "Both the Americans and the French have satellite systems that could be used to provide photography, although it would be something of a needle in a haystack job. The other option is using unmanned aerial vehicles, which can circle areas repeatedly and build up patterns of activity."
American and French commanders operate drones from a newly established base at an airfield in neighbouring Niger. Telegraph Group Ltd
