A night-time curfew is in place in the southern Nigerian oil town of Warri following three days of gun battles between rival ethnic militia groups.
By Dan Isaacs
BBC correspondent in Lagos
The fighting has been intense, and eye-witnesses have described bodies lying in the streets.
But it has so far been impossible to gauge the full extent of the casualties.
The clashes are between two communities, the Ijaw and the Itsekiri, in a dispute over land ownership in the area.
In March, fighting between local groups led to the deaths of dozens of people.
On Sunday, local residents fled for cover as gun battles raged, houses were set ablaze and the army attempted to restore order.
This is a region well-known for its inter-ethnic rivalries. It lies in the heart of Nigeria's oil-rich southern delta region, and historical local tensions have been compounded in recent years by the presence of the multinational companies.
Local militias are often heavily armed and target opposing ethnic groups, perceived to have been granted unfair economic advantages either by the government or by the oil companies.
This instability in the Niger Delta also has a wider implication for Nigeria's ability to meet its oil export quotas.
Following the violence earlier this year, oil companies were forced to shut down many of their facilities in the area, leading to major losses in output and sending tremors through world oil markets.
The multinational companies have so far not commented on whether this latest outbreak of violence is likely to seriously affect crude production.