At least 82 of the nearly 200 schoolgirls who were still missing after a mass Boko Haram abduction in northeast Nigeria in 2014 have been freed, a government official has told Al Jazeera.

The girls were in the town of Banki, close to the Nigeria-Cameroon border, after their release on Saturday, Al Jazeera's Ahmed Idris, reporting from Nigeria's capital, Abuja, said.

"We are told that 82 of the Chibok girls have been freed by Boko Haram," he said.

"According to sources, this may be 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."

Boko Haram seeks to impose a strict Islamic law in Nigeria's mainly Muslim north.

READ MORE: Father of Chibok girls, 3 years on - 'I lost my peace'

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, had been freed.

Negotiations between the Nigerian government and Boko Haram have been ongoing for some time.

Government officials said the talks continued after the release of the 21 girls last year, but in recent months they seemed to have hit a wall amid an escalation in fighting.

Following Saturday's release, Al Jazeera's Idris said there were 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."

READ MORE: One million forced from school by Boko Haram war

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 spurred 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 armed campaign in 2009.