The Nigerian operations of two major international companies have been accused of failing to control oil spills which damage the environment and the health of Nigerians.

In a report released on Thursday, Amnesty International attacks the multinational company, Shell, for too quickly attributing oil spills to sabotage and theft by outsiders. It also accused Agip, a subsidiary of Eni, of not properly controlling its activities.

However, Shell is the main target of the report, which attacks the reliability of the company's investigations into oil spills.

The report says: "Shell has claimed that... oil spill investigations are sound when they are not, that sites are cleaned up when they are not, and that the company is transparent when, in reality, it maintains very tight control over every piece of information - deciding what to disclose and what to withhold."

In a press release accompanying the report, Amnesty acknowledges Shell has improved its investigations since 2011, but says "serious flaws remain, including weaknesses in the underlying evidence used to attribute spills to sabotage."

On Agip, Amnesty says its report raises "serious concerns about the scale of oil spills... which suggest the company is not in control of its operations in the Niger Delta.

"No company can defend more than 400 oil spills a year by referring to sabotage and theft."

No response was immediately forthcoming from Shell Nigeria, but The Guardian in London reported the company's London office as rejecting what it called "unsubstantiated assertions that [Shell Nigeria] have exaggerated the impact of crude oil theft and sabotage to distract attention from operational performance."

Shell said it wanted "greater transparency and independent oversight" in handling oil spills. It also said it could not unilaterally change the process of investigating spills, which also involved regulatory bodies, Nigeria's ministry of environment, the police, state government and local communities.