The chief executive of Agip, a unit of Eni, the Italian oil giant, visited Olusegun Obasanjo, the Nigerian president, on Wednesday to discuss the crisis.

The company's spokesmen said it was unclear what led to the lifting of the two oilfield sieges on Tuesday, but that talks with the attackers were led by the state government and involved elders from nearby villages.

Escalation expected

Industry executives expect violence to escalate in the run-up to Nigeria's general elections in April as local politicians fight turf wars for access to elected office and a share of the government's oil revenue.

Four foreign oil workers - three Italians and one Lebanese - are still being held hostage by a different armed group after an attack on Agip's Brass River export terminal on December 7.

The captors, from the Movement for the Emancipation of the Niger Delta (Mend), have demanded money, jobs and infrastructure for their communities in the remote region of mangrove-lined creeks and swamp in southern Nigeria.

Mend says it has spurned ransom offers and wants the Nigerian government to release two jailed leaders from the region.

It has threatened to keep the men for six months.

Weekly occurrence

Kidnappings and attacks on oil facilities have become an almost weekly occurrence in the world's eighth-largest exporter.

Western oil companies recently evacuated hundreds of dependants of expatriate staff after two car bombings by Mend at oil company compounds in Port Harcourt, capital of Rivers state in the Niger Delta.

Senior industry executives say the rising tide of violence could force them to withdraw from some areas completely.

Shell has already shut down its entire oil operation in the western side of the delta after a series of attacks in February which cut Nigeria's oil output by a fifth.