Gunmen, suspected to be pirates, opened fire on a marine police checkpoint in the Nembe River in Bayelsa state, killing four police officers, officials confirmed on Thursday.

According to reports, the assailants had been on speedboats when they fired at the officers who had been patrolling the waters.

State police spokesman Eguavoen Emokpae told press that “the reason for the attack is still sketchy”.

The region has enjoyed some peace after a 2009 government-instituted Amnesty program led to a significant decline in militant activities in the region. However, Nigeria’s oil-rich south is still described as volatile as criminal gangs remain rather active.

This recent attack on the formation occurred only days after pirates off the Nigerian coast near Port Harcourt, robbed a cargo ship and kidnapped the its captain and engineer.

Militants also claimed responsibility for bombing attacks on Italian owned oil pipelines, promising more attacks in the future.

Nigeria is Africa’s biggest oil producers and foreign firms have pumped oil out of the country for more than 40 years.

Still, impoverished residents of the oil-producing southern region say they are yet to reap the benefits.