Witnesses say four security agents were killed in northeast Nigeria in a roadside bomb explosion.

Reports say the bomb went off Thursday afternoon in the northern city of Damaturu, the capital of Yobe state. A local police officer said security forces exchanged fire with gunmen.

No one immediately claimed responsibility for the attack, but the region has been plagued by violence blamed on the Islamist group Boko Haram.

Explosions have also been reported in other cities in the region, including in Maiduguri, which has been targeted by the radical sect. Boko Haram has been blamed for hundreds of death and a string of bombings in its campaign to implement strict Shariah law in Africa's most populous nation.

The nation's capital Abuja has boosted security in the wake of recent terrorist attacks mostly in the country's north.

In August, Boko Haram was blamed for a suicide bombing at the United Nations headquarters in Abuja, which left at least two dozen people dead.

Nigerian police have arrested 14 suspected Boko Haram members earlier this week and seized guns and explosives from several sites.