Nigeria's president has said he is prepared to negotiate with Boko Haram militants to secure the release of about 200 schoolgirls.
Muhammadu 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 Mr Buhari.
Attempts to negotiate with Boko Haram during the rule of ex-President Goodluck Jonathan failed because officials were talking to the wrong people in the fragmented militant group.
BBC Nigeria analyst Naziru Mikailu says Mr Buhari's offer to revive talks shows that Boko Haram remains a force, despite his claim last week that the group had been "technically" defeated.
There is little prospect of Boko Haram agreeing to negotiate the release of the girls, he adds.
The militants regard the girls as their most invaluable captives and their leader, Abubakar Shekau, said last year that most of them had converted to Islam and had been married off.
Some Nigerians on social media expressed anger at the president for saying the government had no idea where the girls were being held, saying it indicated a failure of the intelligence services.
Mr Buhari took office in May with a promise to defeat the group, and gave the military a deadline of the end of the year to end the six-year insurgency.
Although Boko Haram has been driven out from most of the areas it controlled in north-eastern Nigeria, it has continued to carry out suicide bombings and raids into neighbouring Cameroon, Chad and Niger.
The military has managed to free hundreds of Boko Haram captives in recent months.
However, they did not include any of the Chibok girls.
Boko Haram at a glance:
- Founded in 2002, initially focused on opposing Western-style education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009
- Thousands killed, mostly in north-eastern Nigeria, hundreds abducted, including at least 200 schoolgirls
- Joined so-called Islamic State, now calls itself IS's "West African province"
- Seized large area in north-east, where it declared caliphate
- Regional force has retaken most territory this year