Boko Haram Islamist militants are using children as human bombs and targeting women and girls for particularly horrific abuse, including sexual slavery, United Nations human rights chief said on 1 April.

Zeid Ra'ad Al Hussein told a special session in Geneva that his office had received reports of Boko Haram using children as its first line of attack, as expendable cannon fodders.

"Credible reports indicate that Boko Haram frequently uses children as its first line of attack, as expendable cannon fodder. Bodies of children around 12 years old have been found strewn across such battlefields," Zeid said.

"The group has also repeatedly used young children as human bombs, including a case of a 14-year-old girl carrying a baby on her back who detonated a bomb in a marketplace. These reports, if confirmed by a court of law, they would certainly constitute war crimes and crimes against humanity," he added.

Boko Haram has killed thousands and displaced some 1.5 million people during a six-year campaign to carve out an Islamic state in northern Nigeria.

A joint offensive by Nigeria and its neighbours has succeeded in driving the group from most of the positions they controlled earlier this year, reversing militants gains that forced Nigeria to delay a February election.

Zeid said that appalling atrocities committed by the group created a critical human rights situation not only in Nigeria, but in the whole Lake Chad region.

He said that both children and adults have been abducted by the group on a massive scale.

Women and girls have been enslaved and subjected to sexual violence, forced labour and compulsory conversion, said Zeid, citing reports from witnesses and survivors.

"Since 2009, when the Boko Haram group turned massively to violence, at least 15,000 individuals have been killed. Countless more children, women and men have been abducted, abused and forcibly recruited, and women and girls have been targeted for particularly horrific abuse, including sexual enslavement," Zeid said.

"Villages and towns have been looted and destroyed. Boko Haram has a specific animus against schools - particularly the education of girls - and its attacks have destroyed or severely damaged at least 300 schools, killed numerous students, and ended with the abduction of hundreds of schoolgirls," he added.

According to the reports, retreating Boko Haram militants murdered their so called "wives" - women and girls held in slavery, and other captives as military offensive by Nigeria and its neighbours advanced, said Zeid.

"Responses to these massive violations must be strong, coordinated and principled," he said.

Zeid said he also received information suggesting that violations have also been committed by the security forces of Nigeria and other countries responding to the insurgency, and called for a thorough and transparent investigation.