Shell has denied deploying the troops and said it was not in a hurry to return to Ogoniland, where it was accused of supporting military operations against activists in the 1990s.

Tension has heightened in the Niger delta from where most of Nigeria's oil is pumped since September after the Niger Delta People's Volunteer Force threatened to blow-up oil facilities.

"SPDC (Shell Petroleum Development Company) should within seven days ... withdraw the forces, stop further acts of hostility such as attempts to resume operation," MOSOP president Ledum Mitee said in a statement yesterday.

Mitee added that "SPDC shall be held accountable for the consequences of the non-violent action."

Tension rose in Ogoniland after a spill early this month, from the 24-inch Trans-Niger pipeline which carries crude from oilfields to Shell's Bonny export terminal damaged crops and polluted creeks.

Shell, Nigeria's biggest producer accounting for nearly half of the world's eighth largest exporter's output of around 2.5 million barrels per day, said the pipeline was vandalised.

"It is not Shell's responsibility to deploy security forces. It is the responsibility of government," Shell said yesterday.

"Shell is not in a hurry to return to Ogoniland without due consultation with all stakeholders," a company statement said.

The government has agreed a truce with the Ijaw-based militia led by Mujahid Dokubo-Asari who is pressing for the withdrawal of troops from the impoverished region, autonomy and a bigger share of oil revenues.

Negotiations between the government and the rebels are still going on in the capital Abuja.

The rebels, who had fought sporadic battles with troops since last year, asked expatriates to leave the delta ahead of what Asari described as an "all-out war on the Nigerian state".

Oil prices shot above $50 per barrel for the first time on the threat, as dealers saw a further tightening of already precarious global oil supplies.

Shell which has been blamed for environmental degradation in Ogoniland, is facing a class-action suit that accuses the company of supporting military operations by Nigeria's former government against the Ogoni in the 1990s.

The highpoint of the operations was the execution in 1995 of Ogoni campaigner and writer Ken Saro-Wiwa by the late dictator Sani Abacha's government.