Villagers say Ijaw settlements are under siege
Troops and armoured vehicles are patrolling the streets of the Nigerian oil town of Warri in an attempt to end two weeks of ethnic violence.
An oil worker told the French news agency, AFP that all vehicles were being searched as they entered or left the town.
Ijaw youth leader Behbehnimibor Job told the BBC's Focus on Africa programme that soldiers had burned down four ethnic Ijaw villages, in unprovoked attacks. There has been no independent confirmation of this.
The fighting has led to a 40% cut in Nigerian oil production as oil companies have suspended operations at some facilities.
Nigeria is the world's sixth biggest oil exporter.
An Ijaw leader told AFP that the wharf normally used by fishermen had been cordoned off by the army.
"There is no market, no business life at all. But we are getting used to this, we are suffering in silence," said Oboko Bello, president of the hardline Federation of Ijaw Delta Communities.
The flare-up in tensions centres on demands by Ijaws for greater political representation and more compensation from oil companies operating in the area.
Previously, the army has accused Ijaw militants of attacking the villages of the neighbouring Itsekiri people.
Chevron Texaco has now shut down its main export terminal.
Shell has evacuated four facilities and Total Fina Elf have pulled out of an oil storage farm.
The army say 13 people - including five civilians - have died in the violence.
However, a human rights activist Danka Pueba told AFP she had reports that many more local people had been killed.
Refugees fleeing Ijaw settlements say a state of siege has been imposed on them, with navy gunboats and troops blockading and firing on their villages.
Soldiers have been deployed in the area
Military officials have previously denied attacking civilians, and stressed they used minimum force when possible.
An Ijaw militant activist leader is reported to have repeated threats on Monday to blow up oil facilities unless the Nigerian military ends its incursions into Ijaw villages.
The Ijaw say that as the majority ethnic group in the area they are entitled to greater political representation ahead of next month's elections.
Militant youths have vowed to make the area ungovernable in protest.