Nigeria's President-elect Muhammadu Buhari has vowed to make every effort to free more than 200 schoolgirls abducted by Boko Haram fighters a year ago, but admitted it was not clear whether they would ever be found.
His statement on Tuesday came a day after Pakistani Nobel laureate Malala Yousafzai criticised Nigerian and world leaders for failing to help free the 219 schoolgirls.
The abduction of the girls from a secondary school in Chibok in the country's northeast last April drew international attention to the humanitarian crisis caused by Boko Haram's attempt to establish a caliphate in religiously mixed Nigeria.
A march is expected in the capital, Abuja, on Tuesday to mark the one-year anniversary of the mass kidnapping.
Buhari, whose presidential election win two weeks ago was the first democratic defeat of an incumbent in Africa's biggest economy and most populous nation, said his administration would do everything it could to defeat the armed group, according to the Reuters news agency.
"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.
"My government will do everything in its power to bring them home," added the former military ruler, who said his approach would differ from that taken by President Goodluck Jonathan.
Malala speaks out
Yousafzai, who rose to international fame after being shot by the Taliban for campaigning for girls' rights to education, made the comments on the eve of the first anniversary of the schoolgirls' abduction.
"In my opinion, Nigerian leaders and the international community have not done enough to help you," she said in a letter to the teenagers.
"They must do much more to help secure your release. I am among many people pressuring them to make sure you are freed," she added, calling the girls - abducted on April 14 last year in the northeastern town of Chibok - "my brave sisters".
The United Nations Children's Fund (UNICEF) said on Monday that 800,000 of the 1.5 million people displaced by Boko Haram violence were children.
More than 300 schools have been severely damaged or destroyed between January 2012 and December last year, with at least 196 teachers and 314 schoolchildren killed in that period, it added.
"Children have become deliberate targets, often subject to extreme violence - from sexual abuse and forced marriage to kidnappings and brutal killings," it said in a report, "Missing Childhoods".
Children had also become weapons of war, being forced to fight alongside the fighters and also used as human bombs, it added.
Boko Haram - which launched a rebellion in 2009 - kidnapped 276 girls from their school in Chibok, in Borno state.
Fifty-seven managed to escape soon afterwards but the remainder have not been seen since an appearance in a Boko Haram video in May last year.
Abubakar Shekau, the group's leader, has claimed they have all converted to Islam and been "married off".
Boko Haram opposes Western education and has sought to impose Islamic law in all 36 states of Nigeria, which is roughly equally divided between a predominantly Muslim north and a mainly Christian south.
The group operates mainly in the northeastern Nigerian states of Adamawa, Borno and Yobe, where the government has declared a state of emergency since May 2013.
Source: Al Jazeera and agencies