Islamist insurgents attacked two northern Nigerian state capitals on Monday, hitting a police post and setting off explosions in a market that killed at least five people, witnesses said.

Yobe, Borno and Adamawa states in Nigeria's northeast have been the main targets of Boko Haram militants fighting for five years to carve a radical Islamist state out of Africa's top oil-producing and most populous country.

The insurgents raided the Yobe state capital Damaturu at dawn, setting fire to a mobile police station and shooting at an air force jet that was circling their forces and dropping bombs, according to witnesses.