Twin bomb blasts have killed at least 18 at a Nigerian mosque crowded with people observing pre-dawn prayers, according to a self-defence fighter.

The fighter was among rescuers who evacuated bodies while troops cordoned off the zone.

He said there appeared to be only one suicide bomber, though "we all heard two explosions" at around 5am in a mosque in the Jiddari Polo area of Maiduguri, the biggest city in the north east and birthplace of Boko Haram.

Friday's attack was the latest in a string of assaults blamed on Boko Haram Islamic extremists, who have indiscriminately killed Christians and Muslims they accuse of not following their radical version of Shariah law.

Suicide bombers have killed hundreds in recent months.