Nigerian President-elect Muhammadu Buhari says in a statement that he can't promise to find the 219 Chibok schoolgirls abducted by Islamic extremists one year ago.

He says their whereabouts are unknown and "We do not know if the Chibok girls can be rescued."

Activists are marking the anniversary of the mass abduction from a school in Chibok, a town in northeast Nigeria, with a change in their slogan from "Bring Back Our Girls — Now and Alive" to "Never to be Forgotten."

Pakistani activist Yousafzai Malala, meanwhile, promises the girls scholarships and says they must never lose hope. The 17-year-old Nobel Prize laureate chastises President Goodluck Jonathan and the international community, saying they haven't done enough to rescue the girls.

Boko Haram has kidnapped hundreds more since then.