A Nigerian journalist with links to the Islamist group Boko Haram has claimed only 15 out of the 112 missing Chibok girls are still alive.
The 276 schoolgirls were abducted by Boko Haram exactly four years ago. Many have since been released.
Ahmad Salkida said he had negotiated for the girls' release on behalf of the government, but several opportunities to have them freed had been missed.
The Nigerian government said there was no reason to think some may be dead.
A spokesperson told the BBC the government was still in discussions with Boko Haram to secure the release of the 112 girls who remain missing.
Mr Salkida said the administration of former President Goodluck Jonathan had asked him to negotiate for the release of the Chibok schoolgirls two weeks after they were abducted.
He said he had arranged for a prisoner swap on five occasions, but delays from the then government led to a breakdown in negotiations.
What happened to the 276 kidnapped girls?
Chibok abductions: What we know
The town that lost its girls
Mr Salkida would not reveal the names of the remaining girls, stating that this was the responsibility of the Nigerian government.
The kidnapping led to a global outcry for their release.
Parents marked the fourth anniversary on Saturday by marching with thousands of others to the school in north-eastern Borno state where the girls were abducted in 2014.
Parents whose children had been released wore white during a multi-faith ceremony, the AFP news agency reported - while those whose daughters were still missing wore black.
"Our only prayer is for our girls to be released and returned to us," one parent, Hannatu Daudu, said at the gathering.
"We need to know if they are alive or dead. If they are alive, let them come back to us. If they are dead, let us know so we can at least pray for them and then overcome this grief."