Muhammadu Buhari said he was prepared to swap militants who are in custody for the girls.

The more than 200 girls were seized during a raid on a school in the north-eastern town of Chibok in 2014.

Previous efforts by neighbouring Chad to broker a deal with Boko Haram to secure the girls' release failed.

Speaking on the sidelines of the annual UN General Assembly meeting in New York, Mr Buhari said that splits within the ranks of Boko Haram, which is affiliated to the Islamic State (IS) group, made it difficult to hold talks with them.

"Government had reached out, ready to negotiate, but it became difficult to identify credible leaders. We will welcome intermediaries such as UN outfits, to step in," Mr Buhari said.

The UN has not yet commented on his request.

In August, IS said that Boko Haram leader Abubakar Shekau had been replaced by the younger Abu Musab al-Barnawi.

Mr Shekau denied this, insisting he was still the leader.

The abduction of the schoolgirls led to the #BringBackOurGirls campaign, that was supported by US First Lady Michelle Obama and Pakistani Nobel Peace laureate Malala Yousafzai.

So far only one schoolgirl, Amina Ali Nkeki, has been found.

An army-backed vigilante group stumbled across her in May in the huge Sambisa Forest, close to the border with Cameroon, as she was looking for wood to make a fire.

She was with a baby, and a suspected militant who identified himself as her husband.

Boko Haram had previously said that some of the girls had been enslaved or "married off" to fighters.

Foreign governments, including those from the US and China, had promised to help Nigeria search for the girls when they were abducted while preparing for the school exams in Chibok in April 2014.

Boko Haram has been fighting since 2009 to create an Islamic state, but it has lost most of the territory under its control in the last 19 months following an offensive by a multi-national force.