Nigerian President Muhammadu Buhari has called on the United Nations to help negotiate the release of more than 200 girls kidnapped by terror group Boko Haram two years ago. He said the government was willing to free some detained militants in exchange for the girls, a key demand made by the Islamist outfit.

"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," Buhari said after attending the UN General Assembly in New York.

The 276 girls were kidnapped as they were attending evening school in Chibok, in the restive Borno state, on 14 April 2014. Some of the girls managed to escape and one was rescued in the Sambisa forest earlier this year, but at least 218 are still missing.

Although the Chibok abduction was not the first mass-kidnapping carried out by the terrorists, it was the only one that attracted international outrage. It led to the creation of the global movement Bring Back Our Girls which shone a spotlight on the deadly insurgency of Boko Haram that had previously been under-reported.

In August, Boko Haram released a video purportedly showing some of the Chibok girls. In the video, a Boko Haram militant called on the government to release militants who had been arrested in return for the release of the girls.

The video came as Boko Haram split into two factions after the group's ally, the Islamic State (Isis), appointed Abu Musab Al-Barnawi as its new leader. Abubakar Shekau, who has been leading the group since 2009, denied he had been replaced and vowed to continue his fight.

Who are Nigeria's Boko Haram?

Boko Haram, which has renamed itself Iswap, fights against Western influence in Nigeria and aims to impose its version of Sharia law throughout occupied territories.

The group launches attacks in Nigeria and neighbouring countries in a bid to take control of more territory. Three Nigerian states − Adamawa, Borno and Yobe − have been under a state of emergency since May 2013.

Boko Haram has killed more than 20,000 people since 2009 and was deemed the world's deadliest terror group, surpassing Islamic State in November 2015. Nigeria has also become the world's third-most terrorised country as a result of the group's violent insurgency.

Chibok girls
A video clip, purportedly filmed in December, shows 15 of the girls standing in front of a yellow wall and wearing purple and black hijabs Boko Haram negotiation videos