A video emerged not long after the abduction showing some of the girls wearing hijabs and reciting Koranic verses
Nigeria's military has retracted its statement that some of the schoolgirls abducted from Chibok town in April by Islamist militants had been freed.
Army spokesman Major General Chris Olukolade told the BBC there were girls in military custody, but not those from Chibok as originally thought.
More than 200 girls were seized by Boko Haram fighters from a boarding school in the north-eastern Borno state.
It caused worldwide outrage and sparked a social media campaign.
Protests were organised under the hashtag #BringBackOurGirls, calling on the authorities to do more to free the girls, who had gone to the school in Chibok from surrounding areas to take their final year exams.
Shortly after the abduction, Boko Haram released a video showing more than 100 of them and offering an exchange for prisoners.
In recent days there have been unconfirmed reports that the Nigerian government has been negotiating a deal with Boko Haram to exchange the abducted girls for imprisoned Islamist fighters.
Since a state of emergency was declared in May 2013 in the north-east to end Boko Haram's insurgency, the group's attacks have increased.
Many women and children - including teenage girls - have been taken hostage since then.
Earlier, Gen Olukolade told the BBC there was an ongoing exercise to release the schoolgirls taken from Chibok and that some of them were safe in a military barracks.
But he later called back to retract his statement, saying the authorities were trying to confirm the identities of the girls who are in the custody of the army, but they did not come from Chibok.
Boko Haram's name translates as "Western education is forbidden", and it has carried out raids on schools and colleges, seeing them as a symbol of Western culture.