Nigeria's military says it has agreed a truce with Islamist militant group Boko Haram and the schoolgirls it has abducted will be released.

Nigeria's chief of defence staff, Alex Badeh, made the announcement. Boko Haram has not made a public statement.

The military has struggled to defeat Boko Haram, which has been waging an insurgency in Nigeria since 2009.

The Islamist militant group sparked global outrage six months ago by abducting more than 200 schoolgirls.

They were seized in the north-eastern town of Chibok. The government has been criticised for failing to secure their release.

Nigerian presidential aide Hassan Tukur told BBC Focus on Africa that the agreement followed one month of negotiations, mediated by Chad.

He said Boko Haram had announced a unilateral ceasefire on Thursday and the government had responded.

"They've assured us they have the girls and they will release them," he said.

"I am cautiously optimistic."