Transparency advocates said Royal Dutch Shell and its partners in Nigeria may have exposed shareholders to a high level of risk in a corrupt system.

Global Witness said it was joining Nigerian anti-corruption campaigners in working to expose what they say is an opaque corporate reputation in the country. Global Witness Director Simon Taylor said that, working with Italian energy company Eni, the Dutch supermajor was stained by corruption.

"Shell and Eni exposed their investors to massive risks and have been tainted by this theft from Nigerian citizens," he said in a statement.

Both companies are already under fire for their work in Nigeria. Amnesty International has expressed frustration over the legacy of oil spills in the Niger Delta region. The rights group said the two companies combined reported more than 550 spills in the area last year.

Shell defended its work in Nigeria, saying more than 100 spills reported in the region were the result of sabotage and theft.

Global Witness said Italian prosecutors have launched a formal corruption investigation into offenses tied to Nigeria oil production leases controlled by both companies. The advocacy group said oil production license 245 was sold in the late 1990s for $20 million to a company "secretly owned" by then Nigerian Oil Minister Dan Etete and later sold to Shell and Eni for $1.1 billion.

According to the account from Global Witness, the companies involved denied knowing the money would go to Etete despite their investigations showing otherwise. Neither company offered a statement or responded with comments when queried by UPI.

In early March, Nigerian Petroleum Minister and Managing Director of the Nigerian National Petroleum Corp. Emmanuel Kachikwu said the state oil company, burdened by corruption and revenue losses, will be split up into dozens of distinct companies.