Talks between the federal government and Boko Haram militants on the release of schoolgirls kidnapped from the northeastern town of Chibok in 2014 have been set back by disagreements among their abductors, President Muhammadu Buhari said on Friday.

Buhari made the comments on the eve of the fourth anniversary of the abduction in which Islamist insurgents snatched 276 girls from their school, triggering a global outcry.

Many managed to escape or were released, including 82 girls who were freed in an exchange deal that included several imprisoned senior members of Boko Haram. Despite pressure at home and abroad, government efforts to rescue around 100 of the girls still being held have so far failed.

"Unfortunately, the negotiations between the government and Boko Haram suffered some unexpected setbacks, owing mainly to a lack of agreement among their abductors, whose internal differences have led to a divergence of voices regarding the outcome of the talks," he said in an emailed statement.

The president added that his administration was "doing its very best to free the girls from their captors".

Boko Haram, which seeks to create an Islamic state in northeast Nigeria and split into two main factions in 2016, has killed more than 20,000 people since it launched its insurgency in 2009.

United Nations children's agency UNICEF, in a report published on Friday, said the group had abducted more than 1,000 children since 2013.

Reuters