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Boko Haram's use of child bombers has increased over the last year with one in five suicide attacks now done by children, the UN's child agency says.

 Girls, who are often drugged, were behind three-quarters of such attacks committed by the militant Islamist group in Cameroon, Nigeria and Chad.

It is an 11-fold increase with four attacks in 2014 compared to 44 in 2015 and January 2016.

The change in tactics reflects the loss of territory in Nigeria by the group.

The seven-year insurgency, which has mainly affected north-eastern Nigeria as well as its neighbours around Lake Chad, has left some 17,000 people dead.

Unicef says up to 1.3 million children have been forced from their homes across four countries: Cameroon, Chad, Nigeria and Niger.

The UN report has been released as Nigeria approaches the second anniversary of the kidnapping by Boko Haram of more than 200 girls from their boarding school in the Nigerian town of Chibok.

The report, Beyond Chibok, says that boys abducted and recruited into Boko Haram's ranks are forced to attack their own families to demonstrate their loyalty.

Girls are exposed to severe abuse including sexual violence and forced marriage to fighters.