ABC News yesterday reported that Royal Dutch Shell evacuated staff from two of its major oil production facilities, while Chevron is also believed to have removed employees.

The two companies operate oil fields in the Niger Delta region, accounting for about 60,000 barrels per day of production.

Reuters quoted a Shell spokesperson as saying the violence has not impacted on production.

The newswire reported that the attack happened on Sunday, when gunmen opened fire on a ferry carrying 14 passengers to Kula in the coastal area of Rivers state. Only two passengers survived and were being treated for bullet wounds.

It quoted members of the poverty-stricken community saying the attack was caused by a long-running clash between local factions demanding greater control of oil wealth.

A houseboat at Shell’s Ekulama 2 oil field station was also reportedly set on fire on Monday night.

ABC reported that rebels last year succeeded in slicing Nigeria’s daily crude oil output by more than a quarter, by severing pipelines and threatening foreign oil workers.

The Niger Delta is Africa’s largest oil-producing region.