Some 800,000 children have been displaced due to violence by terror group Boko Haram in north-eastern Nigeria.
According to the UN Children's Fund (Unicef), many of the children have been separated from their families and are subjected to abuse such as rape and forced marriage. Some of them are also being used by the terrorists as combatants and for suicide bomb missions.
Who is Boko Haram?
Boko Haram fights against Western influence in Nigeria and aims to impose its version of Sharia law in the country. The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.
Boko Haram has raided several cities in the north of the country in a bid to take control of more land.
Three states, Adamawa, Borno and Yobe, have been under a state of emergency since May 2013, due to Boko Haram's deadly attacks.
Unicef also warned that the number of child refugees has doubled in the past year and that the number of children not attending schools is now 10.5 million, compared to eight million in 2007.
The report comes a day before the first anniversary of the mass kidnapping from Chibok, where the terrorists abducted some 270 schoolgirls in April 2014. Shortly after, 50 girls managed to escape, but the rest are still missing amid reports that that they are being used as suicide bombers.
Boko Haram has killed thousands of people and captured several territories in north-eastern Nigeria since its insurgency started in early 2000s. The Nigerian army – aided by troops from neighbouring countries Chad, Cameroon, Niger and Benin – has regained control of some of the areas previously controlled.
The army's offensive was launched a few weeks before Nigeria held a presidential election, which was postponed by the then president Goodluck Jonathan in February amid security concerns.
Meanwhile, reports say that Buhari's All Progressives Congress (APC) party won key states — such as Nigeria's richest state, Lagos — from Jonathan's defeated People's Democratic Party (PDP) as voting for state governorship was extended due to widespread violence that prevented Nigerians from going to polling stations.
The APC's victory in Lagos means that for the first time since the end of military rule in 1999, both the governor of this state and Nigeria's president are from the same party.
Opposition candidates are also contesting victory of PDP in the oil-rich southern Rivers state, where at least nine people were killed, election workers abducted and a polling station and a government official's house set on fire.