Eighty three more of the Chibok girls kidnapped in April 2014 in north-east Nigeria by Boko Haram could be released imminently, according to a spokesman for the country's president.

Around 220 girls were taken from their school in 2014 in Chibok in north-eastern 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 after the Red Cross and the Swiss government brokered a deal. They were brought from the north-eastern city of Maiduguri to the capital Abuja to meet state officials.

"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 Reuters.

"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.

Boko Haram has apparently split with a big group moving away from Shekau over his failure to adhere to guidance from the Iraq and Syria-based Islamic State, which in August named Musab al-Barnawi as its new leader for West Africa.

But that appointment was later dismissed in a 10-minute audio clip on social media by a man purporting to be Shekau, exposing divisions within the jihadist group that has plagued Nigeria and neighbours Chad, Niger and Cameroon.

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.

In recent days, the Nigerian army has been carrying out an offensive in the Sambisa forest, a stronghold of Boko Haram.

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. The group still stages suicide bombings in the north-east, as well as in neighbouring Niger and Cameroon.

News of the potential release of 83 more of the girls came yesterday as the twenty-one released last week were reunited with their families during a Christian ceremony held for them in the capital Abuja.

A schoolgirl named Gloria Dame said the girls had survived for 40 days without food and narrowly escaped death at least once. "I was... [in] the woods when the plane dropped a bomb near me but I wasn't hurt," Dame told the congregation.

Speaking in the local Hausa language, she added: "We had no food for one month and 10 days but we did not die. We thank God."

The ceremony was organised by Nigeria's security services which negotiated the girls' release.

The emotional ceremony was interrupted when the girls' relatives arrived and were reunited with them.

"We can all see the joy and emotions of the parents," Information Minister Lai Mohamed said.He added that talks with the Islamists would continue "until all the girls have been released".

Additional reporting by Reuters.