Residents in Nigeria's northern Borno state say attacks by Boko Haram extremists on several villages in the past week have killed at least 27 people.

Boko Haram's leader has urged his fighters to "kill, slaughter and abduct ... and detonate bombs everywhere" in a new video which denies Nigerian government claims that his Islamic extremist group has been crushed.

Boko Haram fighters are overrunning villages near the Nigerian town of Chibok, forcing hundreds of people to flee as they loot and burn in the area, local leaders have said.

A new video allegedly shows the Boko Haram leader disputing claims of his death, and the beheading of a man who identifies himself as the pilot of a missing Nigerian air force jet.