Amnesty International says Anglo-Dutch energy giant Shell has repeatedly manipulated investigations into the cause of oil spills in Nigeria to avoid liability.

In a new report released on Thursday, Amnesty accused Nigeria’s oil companies, and particularly Shell, of blaming criminal sabotage for oil spills where "corrosion, poor maintenance of oil infrastructure and equipment failure” may be the cause.

The investigation process was inherently flawed, according to Amnesty and the Center for Environment, Human Rights and Development (CEHRD), a local group that co-authored the report.

The rights groups noted that probes to determine the extent of Shell's liability for a spill are funded and largely controlled by the company itself, and that the federal body charged with carrying out independent technical analysis of spills lacks the required resources and expertise.

Official conclusions about the cause of spills can be "very subjective, misleading and downright false," they said.

Titled "Bad Information: Oil Spill Investigations in the Niger Delta," the report described the number of spills in the southern Niger Delta region as "staggering.”

It added Shell, the first and biggest petroleum producer in Nigeria, has reported 348 spills since the start of 2012, while the Nigerian Agip Oil Company, a local subsidiary of Italy's ENI, has reported nearly 1,000 over the same period.

But Shell Nigeria responded by rejecting the "unsubstantiated assertions” and repeated claims blaming most spills oil on thieves, who are estimated to siphon between 100,000 and 200,000 barrels of oil a day.