Abubakar Shekau
Boko Haram's leader in Nigeria Abubakar Shekau YouTube

Islamist militant group Boko Haram is likely to have laid land mines and booby traps to prevent the schoolgirls it kidnapped last month from being found.

Darlington Abdullahi, a former air commodore in the Nigerian military told Sky News: "They may have made land mines, one cannot rule that out."

Abdullahi said one way of rescuing the missing girls could be to starve Boko Haram members, who kidnapped almost 300 schoolgirls from their school in the north-eastern Nigerian village of Chibok on 14 April.

"One thing for sure is, even as they go along abducting children, they will also go after food; grabbing food from various angles," Abdullahi told Sky News.

"That is why it's important that if the military from various countries close up on them, the issue of starvation might come in and might even force them to find [exits], and most likely abandon the girls that are still with them within the forest."

Of the 276 girls kidnapped by Boko Haram, 53 managed to escape, leaving 223 still missing. US intelligence sources believe the girls have been split up into smaller groups to make it more difficult to find them.

Rescue efforts have been further complicated after Boko Haram reportedly blew up a second strategic bridge near the location where the girls were first abducted.

Residents said the bridge, which links the states of Borno and Adamawa, was destroyed on Friday. A bridge to Chad was blown up on Monday. Boko Haram also killed an unspecified number of people and abducted the wife and two children of a retired police officer.

International efforts to rescue the girls have stepped up in recent days, with British and US military experts arriving in Nigeria on Friday to assist in the search.

The US said it would also send FBI agents to help trace the missing schoolgirls, while UK ministers are reportedly reviewing the possibility of deploying a Sentinel surveillance aircraft which would operate out of the French military base at N'Djamena in Chad.

Former UK Prime Minister Gordon Brown, the UN's special envoy for global education, also said air and satellite surveillance will be extended beyond Nigeria to neighbouring Cameroon and Niger and Chad in an effort to locate the missing girls.

French President Francois Hollande's office in Paris said a team of French experts arrived in Nigeria on Saturday to assist in collecting intelligence from technical sources and informers, and offer expertise in image analysis.