The girls were released on Saturday and were currently in the town of Banki, close to the Nigerian-Cameroonian border, Al Jazeera's Ahmed Idris, reporting from Nigeria's capital, Abuja, said.

"We are told that 62 of the Chibok have been freed by Boko Haram," he said.

"According to sources, this may be as a result of negotiations - but there have also been some military operations of recent around that area, as well as other parts of northeast Nigeria," Idris added.

Earlier reports had suggested that as many as 82 pupils had been released by Boko Haram.

The armed group seized 276 pupils from the Government Girls Secondary School in the town of Chibok on the night of April 14, 2014, with 57 of them managing to escape in the immediate aftermath of the abduction.

Some of the schoolgirls have since been released, while others managed to escape.

Before Saturday, more than 80, including 21 whose release was negotiated in October 2016, were free.

"These negotiations have been going on for quite some time, and we were told that they continued after the release of the 21 girls last year - however there were some hiccups," Idris said, citing an escalation in fighting in recent months.

Idris said there were also questions around what the Nigerian government might have given to Boko Haram in return for the schoolgirls.

"A lot of people in Nigeria don't believe that Boko Haram will simply release those girls after three years in captivity for nothing in return," he said.

"We were told that Boko Haram was trying to negotiate for the release of some of their top commanders in custody of the Nigerian security services. There are also some reports suggesting that they want some ransom to be paid for some of these girls."

The abduction of the schoolgirls drew international attention to the armed group, while the Nigerian government's failure to act quickly to free the girls sparked a global Bring Back Our Girls movement.

About 2,000 girls and boys have been abducted by Boko Haram since 2014, with many used as sex slaves, fighters and even suicide bombers, according to Amnesty International.

Some 20,000 people have been killed and about 2.3 million displaced since Boko Haram started its violent campaign in 2009.

- News24.com