The UN children's agency says there has been an "alarming" increase in the number of suicide attacks in northern Nigeria, with many of them involving women and children.

There had been 27 attacks so far this year, compared with 26 for the whole of last year, Unicef said in a statement.

Three-quarters of the attacks were carried out by female bombers, some as young as seven, it added.

Militant Islamist group Boko Haram is waging an insurgency in Nigeria.

BBC Nigeria analyst Naziru Mikailu says Boko Haram may be increasingly resorting to suicide attacks, following its loss of territory to regional forces.

'Exploited'

Nigeria's army said in March that it had recaptured all cities and towns from the group, which has pledged allegiance to Islamic State militants fighting for a global caliphate.

Unicef said it did not believe that the girls carried out the bombings willingly.

"Children are not instigating these suicide attacks; they are used intentionally by adults in the most horrific way," it said. "They are first and foremost victims - not perpetrators."

Some of the girls had been abducted and were being exploited by militants who knew that they were less likely to be checked when entering target areas such as busy markets or bus stations, the statement added.

Our correspondent says there is suspicion that some girls are duped into being bombers, thinking they are carrying parcels when they are, in fact, explosives.

More than 15,500 people have been killed during the six-year insurgency.

About 1.5 million people have been displaced and hundreds more abducted by Boko Haram.

Earlier this month, the military rescued nearly 300 women and children who were being held by the group in its forest hideout in north-eastern Nigeria.

Among those still being held captive are 219 girls who were taken from their boarding school in Chibok town in April 2014, sparking global outrage.