Nigeria's president, Muhammadu Buhari, has said that he was prepared to negotiate with Boko Haram militants to secure the release of about 200 schoolgirls.
Buhari said that if a credible Boko Haram leadership could be identified then he was prepared to talk with them without preconditions.
But he said he had no intelligence on the girls' whereabouts or their health.
Boko Haram seized the girls from their dormitories in the north-eastern town of Chibok in April 2014.
"If a credible leader of Boko Haram can be established and they tell us where those girls are, we are prepared to negotiate with them, without any precondition," said Buhari.
Previous attempts to negotiate reportedly failed because officials were talking to the wrong people in the fragmented militant group.
Hundreds of Boko Haram captives have been freed in recent months as Nigeria's military has driven the militants out of territory they held and into a forest enclave.
However, they did not include any of the Chibok girls.