"A total of 180,000 barrels a day has been temporally shut in and we don't know when it will be back," Shell spokeswoman in London Caroline Wittgen told AFP on Tuesday.

A spokesman Shell in Nigeria, Precious Okoloko, confirmed the leak to AFP but declined to comment on the actual quantity of oil shut in following the incident.

"The spill occurred on Friday along our pipeline in Krakrama community, in Rivers State. We quickly mobilised containment and investigation teams to the scene," he said.

"Unfortunately, the community has not granted our teams access to the scene of the spill. We still do not know the exact extent of the spill. We are still trying to determine what we have shut in," he added.

The news was a factor pushing crude futures above 75 dollars per barrel in London and New York trading on Tuesday. World oil prices were supported also by violence in the Middle East and concerns over refinery shut-downs in the United States, dealers said.

The Shell leak came as Nigeria was already experiencing a cut of about 20.0 percent in its oil production, or 500,000 barrels per day, owing to militant unrest in the Niger Delta.

The latest disruption has cut the country's oil production by more than 26.0 percent. Under normal circumstances, Nigeria's daily output totals 2.6 million barrels per day.

Last week, a parliamentary team that visited a Shell oil spill site in Jesse, in southern Delta State, gave the Anglo-Dutch company a week to clean up the polluted environment and pay compensation to those whose livelihood has been affected by the disaster.

Aggrieved militants and communities in oil-rich southern Niger Delta have since the beginning of this year intensified violent actions against foreign oil workers and installations.

Some 25 security personnel have been killed since January when separatist militants seeking local control of Nigeria's multi-billion-dollar oil and gas resources launched attacks on oil firms and personnel in the region.

At least 32 expatriate oil workers have also been kidnapped but all have been released unharmed after days or weeks in capitivity.

The unrest has led to a 25-percent cut in oil exports from Nigeria, Africa's largest producer and the world's sixth largest crude exporter.

Niger Delta militants on Tuesday accused US oil firm Chevron of using the Nigerian military to attack ethnic Ijaw villagers and warned of reprisals if the attacks continued.

"If this incessant harassment is not stopped forthwith, we are not going to hesitate to descend mercilessly on ChevronTexaco Nigeria's interests anywhere in Nigeria," the Movement for the Emancipation of the Niger Delta (MEND) said in a statement to AFP.

"From the cry of the people, it has become unbearable, hence this warning to the international community and the federal government to immediately call ChevronTexaco to order," it said.

It said a similar attack by the Nigerian military in February forced the militants to kidnap nine expatriate oil workers who were released after several days in capitivity.

There would be more trouble "if the military does not stop its aggression against Ijaw ethnic nationalities in the waterways," it warned.

Two weeks ago, four navy officers escorting a convoy of boats working for Chevron were killed by separatist militants near Chanomi creek in the troubled region.

No group claimed responsibility for the attack.