The action has halted more than a fifth of the country's oil output of two million barrels per day.

But the demonstrators, many of them ethnic Ijaws, have vowed to continue attacking Shell sites until they get a new local government.

They say Nigeria's military government is siding with the rival Itsekiri group in the area.

In the latest attacks the protesters seized two helicopters from a Shell-owned helipad in the oil rich Niger River delta.

Demonstrators also took control of a nearby oil rig belonging to a foreign contractor working for the company.

Armed youths had earlier seized more than 10 oil relay stations in the same region.

The Ogoni tribe crisis came to a head with Ken Saro-Wiwa's execution in 1996

The protests are the first in several years to have had such a drastic effect on output.

They are part of an upsurge in violence in the Niger Delta where impoverished communities are demanding a greater share of the oil wealth that accounts for more than 90% of Nigeria's export income.

Youth groups say their action is aimed at the government and what they call their exclusion from their country's political process.

Although rich in oil, the Niger River delta states are among the poorest and most neglected in Nigeria.

Shell said it had informed buyers it could no longer guarantee availability ''because of the shutdown of 378,000 barrels per day of crude production from 15 flow stations.''

The protests started four days ago when large groups, armed with automatic weapons, began boarding the flow stations which pump oil to export terminals.

Agip also targetted

At least 500,000 barrels of oil a day is now being lost in Nigeria - one of the world's biggest oil-producing nations.

Protesters have also targetted the oil company Agip which says it is losing 130,000 barrels a day.

The BBC's correspondent in Lagos, Hilary Andersson, says many inhabitants of the densely-populated delta can see multi-million dollar oil installations from their makeshift homes where there is often no electricity or public water supply.

Since a crisis involving the Ogoni tribe came to a head in 1996, when community leader Ken Saro-Wiwa was executed by the military government, the people of the delta have become more politicised and the security situation now is said to be out of hand.