Nigerian President Muhammadu Buhari has asked the United Nations for help in rescuing the Chibok schoolgirls from Boko Haram captivity.

It has been two and a half years since the militant Islamic group group kidnapped 276 female students from a boarding school in Chibok. Some of the girls escaped immediately after the kidnapping, and a handful have been rescued, but 217 girls remain missing despite international calls to increase efforts to rescue them. The kidnapping launched the international hashtag campaign #BringBackOurGirls.

While attending the UN General Assembly in New York this week, Buhari said he would accept the help of UN intermediaries in negotiating the girls’ release with Boko Haram leaders.

“President Buhari said the Nigerian government was willing to bend over backwards, to get the Chibok girls released from captivity,” Femi Adesina, a spokesman for Buhari, said in a statement.

Read More: Two Nigerian Girls Abducted by Boko Haram Have Been Rescued

Buhari said his government had trouble finding Boko Haram leaders with whom they could negotiate.

"The challenge is in getting credible and bona fide leadership of Boko Haram to discuss with," Buhari said. "The split in the insurgent group is not helping matters.”

Nigerian Information Minister Lai Mohammed said last week that the government had tried to negotiate the girls’ release three times but negotiations collapsed. He told The Guardian that a recent change in Boko Haram’s leadership meant the government is “dealing with a group that can change the goalposts” of negotiations at any time.

A video was released in August showing 50 of the Chibok girls alive and being held captive, prompting their family members and many advocates to call on Buhari to rescue them. One of the schoolgirls was found wandering in a forest where Boko Haram is headquartered in May and was rescued.

Pres @MBuhari#CountDownToDay900 since OUR #ChibokGirls abduction.
WE WANT ACTION!
WE WANT RESULTS!
NO MORE EXCUSES!!
NO MORE DELAYS!!!

— Oby Ezekwesili (@obyezeks) September 21, 2016

Read More: Thousands of Children Survived Boko Haram in North East Nigeria — But Now They’re Starving to Death

Many of the girls are believed to have been forced into marriage and motherhood by the fighters, according to the Associated Press.

According to the AP, the video also instructed the government to deal with a journalist to arrange a prisoner swap. The journalist, Ahmad Salkida, was detained by the Nigerian government but released.

According to The New York Times Boko Haram has killed thousands of people and uprooted 2.6 million people across four countries