Young girls are increasingly being used as suicide bombers by Nigeria's Boko Haram Islamist insurgents – and there are increasing fears that some of the abducted Chibok schoolgirls might be among them.

The 276 students were kidnapped in April last year and inspired a global campaign for their release, with the 'Bring Back Our Girls' slogan widely shared on social media. Around 219 are thought to be still missing and are believed to be held together in a secret location.

Recent atrocities perpetrated by Boko Haram, which is under pressure from international as well as Nigerian forces, include the May 16 attack on a bus station in Damaturu in northeastern Nigeria. A girl of about 12 detonated explosives hidden under her clothes, killing seven people and injuring 31.

In November, five people were killed and at least 32 wounded when two girls exploded bombs in a crowded market in Maiduguri, the capital of Borno state.

A raid in March on Damasak, also in Borno, saw Boko Haram fighters burn down houses and abduct children including Aisha (9) and Falmata (13). Their brother Bukkar told The Daily Beast: "They set our house on fire and walked through the streets kidnapping children who were under 15 years of age and killing those who were above that age. They were most interested in little girls, whom they plan to use as suicide bombers."

Yusuf Mohammed, who works with young people affected by trauma in Maiduguri, said: "Militants feel it is easier to intimidate and brainwash young girls than adult women. Besides, these girls come cheap, and most of them are extremely loyal."

Last month, rescuers who freed nearly 300 woman and girls from Boko Haram in the Sambisa Forest were fired on by some of those they had come to help in a sign that they had been successfully indoctrinated by their captors.

The Chibok schoolgirls are believed to be among hundreds of women and girls kept in camps in the Sambisa.