Nigerian president-elect Muhammadu Buhari has said he cannot promise to find 219 schoolgirls abducted by Islamic extremists a year ago.
He said their whereabouts are unknown, adding: "We do not know if the Chibok girls can be rescued."
Activists have marked the anniversary of the mass abduction from a school in a town in north-east 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, has promised the girls scholarships and says they must never lose hope.
The 17-year-old Nobel Prize laureate criticised president Goodluck Jonathan and the international community, saying they have not done enough to rescue the girls.
Boko Haram has kidnapped hundreds more since then.
"As much as I wish to, I cannot promise that we can find them," Mr Buhari said in a statement.
The failure to rescue the girls elicited condemnation of Mr Jonathan's government and the Nigerian military, which has repeatedly made false statements about the girls and continues to make hollow promises to bring them home.
Those failures contributed to Mr Jonathan's thrashing at the polls on March 28 by Mr Buhari, a former military dictator who says he is a convert to democracy and has promised a new approach.
At least 2,000 women and girls have been abducted by Boko Haram since the start of last year, and many have been forced into sexual slavery and trained to fight, Amnesty International said in a report marking the anniversary.
Hundreds of boys and young men have also been kidnapped and forced to fight with the extremists, or slaughtered for refusing to do so, it said.
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