Boko Haram militants have released 82 schoolgirls out of a group of more than 200 who they kidnapped from the northeastern town of Chibok in April 2014, officials say.

The girls were released through negotiations with the government, one official said, asking not to be named.

A military source said the girls were currently in Banki near the Cameroon border for medical checks before being airlifted to Maiduguri, the capital of Borno state.

Earlier there was confusion over how many girls had been released, with one report saying up to 50 were free.

The kidnapping was one of the high-profile incidents of Boko Haram's insurgency, now in its eighth year and with little sign of ending. About 220 were abducted from their school in a night-time attack.