A Nigerian government-backed vigilante group has freed 833 children it had recruited to fight Boko Haram militants, the UN children's agency Unicef has said.

In a statement, Unicef hailed the release as a "significant milestone in ending the recruitment and use of children, but many more children remain in the ranks of other armed groups in either combat or support roles".

The children - some as young as 11 - were freed by the Civilian Joint Task Force, formed in north-eastern Nigeria to help government troops defeat the militant Islamists.

"As of today, a total of 1,469 children (1,175 boys and 294 girls) associated with the Civilian Joint Task Force have been identified within the city of Maiduguri," Unicef added.

The US ambassador to Nigeria, W Stuart Symington, spoke at an event to welcome the freed children.