The Islamic State-allied faction of Boko Haram, which last week freed 21 of more than 200 Chibok girls kidnapped in April 2014 in northeast Nigeria, is willing to negotiate the release of 83 more of the girls, the president's spokesman says.
Around 220 girls were taken from their school in 2014 in Chibok in northeastern Borno state, where Boko Haram has waged a seven-year insurgency aimed at creating an Islamic state, killing thousands and displacing more than two million people.
A faction of the militant group released 21 of the girls on Thursday (local time) after the Red Cross and the Swiss government brokered a deal.
"These 21 released girls are supposed to be tale bearers to tell the Nigerian government that this faction of Boko Haram has 83 more Chibok girls," Garba Shehu, spokesman for President Muhammadu Buhari, told the Thomson Reuters Foundation by phone on Sunday.
"The faction said it is ready to negotiate if the government is willing to sit down with them," said Shehu, adding that the state is prepared to negotiate with the branch of Boko Haram.
The Islamic State-allied splinter group said the rest of the kidnapped Chibok girls were with the part of Boko Haram under the control of figurehead Abubakar Shekau, according to Shehu.
Information Minister Lai Mohammed on Thursday denied reports that the state had swapped captured Boko Haram fighters for their release and said he was not aware if any ransom had been paid. He said a Nigerian army operation against Boko Haram would continue.
The militants controlled a swathe of land around the size of Belgium at the start of 2015, but Nigeria's army has recaptured most of the territory.