text: Boko Haram fighters  have been murdering dozens of young women and girls they had taken as 'wives' and using children as 'expendable cannon fodder', the U.N.'s human rights chief said today. As the Islamist militant group has retreated from parts of northeastern Nigeria, 'gruesome scenes of mass graves and further evident signs of slaughter,' has been brought to light, Zeid Raad al-Hussein told a special session of the U.N. Human Rights Council in Geneva. Boko Haram's reported use of children as shields and human bombs would, if confirmed, constitute war crimes and crimes against humanity, he said. Scroll down for video . 'War criminals': Zeid Ra’ad Al Hussein, U.N. High Commissioner for Human Rights, told the U.N. Human Rights Council in Geneva that if reports are true, BGoko Haram have committed crimes against humanity . Boko Haram leader Abubakar Shekau (pictured) and his group of insurgents have reportedly been murdering women forced into marriage with fighters, and using children as 'expendable cannon fodder' This comes in response to last week's reports that hundreds of women and children had been seized from Nigerian primary schools by Boko Haram militants to be used as 'human shields'. The Islamic militants took as many as 500 adults and children from primary schools in the north-eastern town of Damasak in late February, Mike Omeri, spokesman for the fight against Boko Haram, confirmed. 'Boko Haram rushed to primary schools they took children and adults that they are using as shields to protect themselves from the menacing advance of troops,' he added. There has also been eye-witness reports of the insurgents slaughtering dozens of Nigerian women who had been forced to marry Boko Haram fighters, before a battle with government troops in the northeast town of Bama. Witnesses said the women's Islamist militant 'husbands' feared they would be killed by advancing troops, and murdered their 'wives' to prevent them from subsequently marrying soldiers or 'infidels'. A member of the Nigerian army covers his nose as human remains are discovered at the former prison of Bama,  used by Boko Haram to hold any opponents, many of who were killed when the militants fled the town . Civilians who fled the fighting in Bama and the surrounding areas in recent days walk at a makeshift camp for displaced people on the outskirts of Maiduguri . Mr Al-Hussein, U.N. High Commissioner for Human Rights, also told the council that there are 'persistent and credible reports' of serious rights violations by Nigerian and other security forces responding to Boko Haram. Mr Al-Hussein, son of Prince Ra'ad bin Zeid of Jordan,  called for 'thorough and fully transparent investigations' by authorities. Zeid Ra’ad Al Hussein, U.N. High Commissioner for Human Rights, is the son of Prince Ra'ad bin Zeid of Jordan, also head of the Royal Houses of Iraq and Syria, . However, the UN does not permit the use of royal or other titles by its officials in the context of their official work. Boko Haram has been waging a guerilla war in northern Nigeria and also attacked towns in neighboring countries, prompting nations in the region including Chad and Niger to put together a force to combat them. The newly elected president of Nigeria, General Muhammadu Buhari, who beat incumbent Goodluck Jonathan in a landslide win last night, has promised to 'stamp out' the insurgency . In Kano state, where Boko Haram's 'war' on the civilian population has been most prominent, Buhari delivered a crushing defeat to Jonathan, winning 1.9 million votes for Jonathan's 215,800.
target: Boko Haram accused of war crimes by U.N. human rights chief .
  Militants in Nigeria accused of murdering 'wives' during retreat .
  Reports also say they have used children as 'expendable cannon fodder'