Nigeria's president-elect Muhammadu Buhari on Tuesday cautioned he could not make promises on the return of 219 schoolgirls kidnapped by Boko Haram, as the country marked the first anniversary of their abduction.
The comments by Buhari, who takes office on May 29, stand in contrast to outgoing President Goodluck Jonathan, who has repeatedly said the girls will be found, and the military, which said last year it knew where the teenagers were being held.
Events were taking place in Nigeria and around the world to mark the first anniversary of the abduction, which Amnesty International said was one of 38 since the beginning of last year that had seen at least 2,000 women taken by the militants.
The UN and African rights groups also called for an end to the targeting of boys and girls in the conflict, which has left at least 15,000 dead and some 1.5 million people homeless, 800,000 of them children.
Buhari said there was a need for "honesty" in his new government's approach to the girls' abduction, with nothing seen or heard from the students since last May when they appeared in a Boko Haram video.
"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."
The focus of the one-year commemoration was on Nigeria's capital, Abuja, where a vigil demanding the girls' immediate release has been held almost every day since they were kidnapped.
In New York, the #BringBackOurGirls campaign said the Empire State Building would be lit in its colours of red and purple, to symbolise an end to violence against women.
Prayers, candlelit vigils and marches have been held or are planned but no event was planned in Chibok itself.
Chibok elder Enoch Mark, whose daughter and niece are among the captives, said the town was still in "perpetual fear" of Boko Haram, despite the presence of troops.
"The last one year has been a period of sadness, emotional torment and hardship. It has been one year of mourning. We are a bereaved community that has lost 219 daughters," he told AFP.
He added: "Our hope in finding our girls is now in Buhari. We hope we will soon see our girls if they are alive or at least their corpses if they are dead.
"I personally know what Buhari did as brigade commander for the northeast in 1975. We all know how in 1984 he crushed the violent Maitatsine sect, which is similar to Boko Haram".
Boko Haram fighters stormed the Government Secondary School in Chibok on the evening of April 14 last year, seizing 276 girls who were preparing for end-of-year exams. Fifty-seven escaped soon afterwards.
Boko Haram leader Abubakar Shekau has since said the remainder have all converted to Islam and been "married off".
The mass abduction brought the brutality of the Islamist insurgency unprecedented worldwide attention and prompted a viral social media campaign demanding their immediate release.
Nigeria's government was criticised for its initial response to the crisis and was forced into accepting foreign help in the rescue effort after a groundswell of global outrage.
In a new report published on Tuesday, Amnesty quoted a senior military officer as saying the girls were being held at different Boko Haram camps, including in Cameroon and possibly Chad.
Testimony gathered by Amnesty from women and girls who escaped the militants said they were subject to forced labour and marriage, as well as rape.
#BringBackOurGirls organisers thanked supporters across the world, from ordinary men, women and children to public figures such as US First Lady Michelle Obama and Nobel laureate Malala Yousafzai.
The girls have become "the symbol for the defence of the dignity and sanctity of human life, of the girl child, women, for all those oppressed, repressed, disadvantaged, hurting, unsafe," they said.
Malala, who was shot and nearly killed by the Pakistani Taliban for advocating girls' education, on Monday published an open letter to the Chibok girls, describing them as "my brave sisters".
The 17-year-old criticised Nigerian and world leaders for not doing enough to help secure their release and called the girls "my heroes".
Twenty-one of the 57 girls who escaped are currently studying at the American University of Nigeria.