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 the next year, including 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.

Their abduction sparked the global campaign Bring Back Our Girls, but none of the girls have yet been found.

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.

A 17-year-old girl who was abducted and is living with her baby in a camp in the Nigerian city of Maiduguri told Unicef that she refused to marry despite death threats.

"Then they came for me at night. They kept me locked in a house for over a month and told me: 'Whether you like it or not, we have already married you.'"

She says she has often been ostracised since her escape by other women who accuse her of being a "Boko Haram wife".

The girls who are used by Boko Haram for bombings are often drugged and then explosives are strapped to their bodies.

According to the report, Cameroon has the highest number of child suicide attacks, involving children who are as young as eight.

Boko Haram, which means "Western education is forbidden", has often targeted schools during its insurgency.

Unicef says more than 1,800 schools have been closed, damaged, looted, set on fire or used to shelter those left homeless in north-eastern Nigeria and Cameroon.

Children have also been separated by from their parents during the conflict, with Unicef putting the figure at more than 5,000.

Over the last 15 months, most of the areas controlled by the militants have been retaken by a multi-national force and the militants are now thought to operate from hideouts in Nigeria's Sambisa forest, near the border with Cameroon.

The report says the frequency of all suicide bombings increased from 32 in 2014 to 151 last year, giving this breakdown:

- 89 in Nigeria

- 39 in Cameroon

- 16 in Chad

- Seven in Niger.