Nigeria's president-elect Muhammadu Buhari on Monday denounced Boko Haram's claim to be a religious group, slamming its attacks on civilians and vowing a hard line against the Islamist militants.

"The fraud called Boko Haram can be defeated by denying it a recruitment base. No religion allows for the killing of children in school dormitories, in markets and places of worship," he said.

"They have nothing to do with religion. They are militants and we are going to deal with them as we deal with militants."