Hundreds of villagers have been fleeing

Army spokesman Colonel Onwuamaegbu Chukwuemeka said a military task force had stopped Ijaw militants attacking oil installations and rival local communities.

But the three main oil companies in the region have been further cutting back their production in the coastal swamps.

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 combined total loss of production now stands at more than one-third of Nigeria's normal output.

The army say 13 people - including five civilians - have died in the violence.

However, a human rights activist Danka Pueba told the French news agency she had reports that many more local people had been killed.

Fears

The BBC's Dan Isaacs in Lagos warns that with existing uncertainty in the oil markets as a result of the war in Iraq, this trouble in the world's sixth oil exporter will be causing increasing concern.

Nigeria is Africa's largest oil producer

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.

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.

They are also demanding a greater share of the oil wealth that comes, they say, from their traditional lands.