Boko Haram's use of child bombers has increased over the last year with one in five suicide attacks 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 the next year, including January 2016.

The change in tactics reflects the loss of territory inNigeria 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.

It is nearly two years since more than 200 girls were kidnapped from their school in Chibok in Nigeria.

It sparked the global campaign Bring Back Our Girls, but none have yet been found.