The Ijaw they are discriminated against
The number of casualties from the recent violence in the Delta port of Warri is much higher than previously believed, says the Red Cross in Nigeria.
After three days of relative calm, the Nigerian Red Cross says it has been able to assess the situation more accurately and it believes that about 100 people were killed and 1,000 injured.
It says the fighting between militias of the local Ijaw and Itsekiri people also drove several thousand residents of the city from their homes.
Army troops and riot police were deployed in the city, where authorities said that they had secured a cease-fire on Thursday, between the warring groups.
The violence between the Ijaw and Itsekiri people, in the oil-rich Delta region of Nigeria - has been the worst since March, when several multi-national oil companies were force to halt their operations.
Delta State governor James Ibori, who led a peace mission to Warri on Wednesday, said he understood that some communities felt excluded from the economic benefits of the oil rich delta region but said the solution lay in dialogue and not armed confrontation.
The Ijaw perceive the Itsekiri to have been favoured by both the government and multi-national oil companies operating in the area.
But the BBC's Dan Isaacs says many people in Warri believe the real dispute is about which of the heavily armed groups controls the illegal trade in stolen crude oil.
Industry analysts estimate that at least 100,000 barrels of oil are diverted from official exports every day - rich pickings for the criminal underworld, using ethnic rivalries as an excuse for gang warfare.
Shell has advised its office staff not to turn up for work for the time being, but said oil-production had not been disrupted.
Following the violence in March, the oil companies evacuated most of their operational staff from the area, drastically cutting back production.
Some of that output has subsequently been restored, but these latest clashes are once again causing concern over the impact they will have on Nigeria's oil industry.