Boko Haram killed about 15,000 since 2009, including women and girls, and dumped the bodies of many of them in mass graves, the UN’s human rights chief said.
Zeid Raad al Hussein said that evidence of slaughters and hasty burials had been discovered by Nigerian troops as they recaptured parts of northeastern Nigeria in recent weeks.
Last week soldiers from Nigeria and Chad reported finding at least 70 bodies with their throats slit under a bridge in the town of Damasak when it was liberated from the militants.
Pierre Buyoya, a representative of the African Union, called Boko Haram "a global threat," that poses a danger even outside the region where they are active. Their insurgency has claimed the lives of at least 15,000 people since 2009, said the UNHCR head.
He warned that in turn Boko Haram was becoming suspicious of ethnic groups perceived to support the Nigerian government, thus risking a rapid escalation of the conflict.