Heavy gunfire has erupted for a fourth day in Southern Nigeria where ethnic militias have been battling for power in the oil-rich region.

Witnesses say they heard gunfire again Monday in the town of Warri after the dusk to dawn curfew ended. They say it is not clear whether the new shooting came from ethnic militias or security forces.

There is no official death toll but local media reports more than 20 people have been killed. Hundreds of people fled the town Sunday as street battles raged and dozens of houses were set on fire.

A resident told the AFP news agency that the situation in the town is a "little less than hell."

The fighting is the worst since March, when fighting between Ijaw and Itsekiri militants forced oil firms to abandon key production facilities. The conflict has gone on for years and has claimed hundreds of lives.

The origin of the latest clashes is not yet clear, but competition for the region's oil wealth as well as land and political disputes are often the trigger for violence.