The Nigerian schoolgirls who remain captives of Boko Haram militants "definitely" face the danger of being raped, a senior United Nations official has said.

The Archbishop of Canterbury Justin Welby has warned of the difficulties of negotiating with armed groups in Nigeria like Boko Haram.

The Most Reverend Welby worked in West Africa for several years in oil exploration and has first hand experience of trying to negotiate with "extraordinarily violent and very unpredictable" extremist groups.

He also that full-scale military involvement would bring about the "immense danger" of unifying disparate groups as many would see such an operation as an attack on Islam.