Nigeria's president-elect Muhammadu Buhari on Tuesday cautioned the he could not make promises on the return of 219 schoolgirls kidnapped by Boko Haram exactly a year ago.

"We do not know if the Chibok girls can be rescued. Their whereabouts remain unknown. As much as I wish to, I cannot promise that we can find them," he said in a statement.

"But I say to every parent, family member and friend of the children that my government will do everything in its power to bring them home."