A witness says two girls have blown themselves up near a crowded mosque in northeast Nigeria's biggest city, and many people are feared dead.

It was too early, however, to get an exact number killed.

It is the fourth suicide bombing this month in Maiduguri, which is the birthplace of the Boko Haram Islamic extremist group.

Fishmonger Idi Idrisa says one teenager exploded as she approached the mosque crowded with people doing afternoon prayers during the holy month of Ramadan.

He says the second teen appeared to run away and blew up further away, killing only herself.

Boko Haram has stepped up attacks since Nigeria's new President Muhammadu Buhari announced the military command center is moving to Maiduguri, the capital of Borno State.