Rights group Amnesty International said Friday investigations into Shell oil spills in Nigeria were a "fiasco," alleging the company repeatedly blamed sabotage in an effort to avoid responsibility.

"No matter what evidence is presented to Shell about oil spills, they constantly hide behind the 'sabotage' excuse and dodge their responsibility for massive pollution that is due to their failure to properly maintain their infrastructure," Audrey Gaughran, director of global issues at Amnesty, said in a statement.

She said that "the investigation process into oil spills in the Niger Delta is a fiasco," referring to the oil-producing region that is home to Africa's largest crude industry.

The London-based rights group accused the Anglo-Dutch oil major of ignoring evidence that the latest spill in the Delta's Bodo Creek area, discovered in June, was caused by pipeline corrosion.

Bodo Creek saw two major oil spills in 2008 over which the Anglo-Dutch petroleum giant is being sued in a London court by 11,000 Bodo residents.

An official from Shell's Nigerian subsidiary told AFP the company was not ready to comment on the latest allegations.

In the statement, Amnesty said it hired the US company Accufacts to examine pictures of the Bodo Creek pipeline over the June spill.

According to Amnesty, the company said it noticed a "layered loss of metal on the outside of the pipe," which is "a very familiar pattern" consistent with corrosion.

"Shell have said locally that the spill looks like sabotage, and they completely ignore the evidence of corrosion," said Stevyn Obodoekwe of the Centre for Environment, Human Rights and Development, which co-authored the Amnesty statement.

"This has generated a lot of confusion and some anger in the community," he added.

Sabotage is a worsening problem in the Delta, where oil thieves often blast into pipelines and siphon off crude for sale on the black market.

Some estimates suggest Nigeria loses 150,000 barrels of crude per day to oil theft, known locally as bunkering.

Shell has admitted liability in the 2008 disaster in Bodo, although there remain significant disagreements over the amount of oil that poured into the creeks.

Claims of the amount spilled have ranged from 1,640 barrels to more than 60 times that amount.

Nigeria last month hit Shell with a $5.0 billion (four billion euro) fine over a December leak at the Bonga oilfield that spilled roughly 40,000 barrels of crude into the Gulf of Guinea.

The company is contesting the fine and has insisted there was no basis for it since it had acted quickly to contain the spill.

A landmark UN report last year set out scientific evidence for the first time of devastating pollution in Ogoniland, part of the Niger Delta and where Bodo is also located.

It said years of pollution may require the world's biggest ever clean-up, while detailing urgent health risks, especially badly contaminated drinking water.

Shell faced criticism in the report, which said "control and maintenance of oil field infrastructure in Ogoniland has been and remains inadequate ..."