Lagos, Nigeria (CNN) -- Restoration of Nigeria's environmentally devastated oil-producing Niger Delta region could take up to 30 years, cost $1 billion and become the largest cleanup operation in history, the United Nations said Thursday.
A landmark report from the U.N. Environmental Program (UNEP) concluded that pollution from more than 50 years of oil operations in Nigeria's Ogoniland region is more far-reaching than thought. The assessment, commissioned by the Nigerian government and funded by oil giant Shell, comes on the heels of the company admitting liability for two spills in Nigeria.
Nigeria's Niger Delta, the world's third largest wetland, is diverse and rich with mangroves and fish-rich waterways. But oil drilling has turned it into one of the most oil-polluted places on Earth with more than 6,800 recorded oil spills, accounting for anywhere from 9 million to 13 million barrels of oil spilled, according to activist groups.
But the environmental disaster has never received the kind of attention paid to last year's oil catastrophe along the U.S. Gulf Coast. Amnesty International, which has researched the human rights impacts of pollution in the Delta, said people in the region have experienced oil spills on par with the Exxon Valdez disaster every year for the last half century.
Many residents make their livelihoods from fishing and are dependent on the polluted mangroves and creeks.
"This report proves Shell has had a terrible impact in Nigeria, but has got away with denying it for decades, falsely claiming they work to best international standards," said Audrey Gaughran, the monitoring group's global issues director.
In a statement issued Thursday night, Shell said the two spills to which it has admitted liability amounted to about 4,000 barrels. The statement from Mutiu Sunmonu, managing director of Shell's Nigerian subsidiary, blamed most of the spills on sabotage or attempts to steal oil.
"It is regrettable that any oil is spilt anywhere, but it is wildly inaccurate to suggest that those two spills represent anything like the scale which some reports refer to," Sunmonu said. He called on the Nigerian government "to end the blight of illegal refining and oil theft in the Niger Delta."
In a 2009 report, Amnesty cited independent environmental and oil experts in estimating that between 9 million and 13 million of barrels had leaked in the five decades of oil operations in the Delta. It also quoted U.N. figures of more than 6,800 recorded spills between 1976 and 2001.
Oil companies operating in the region believe the figures are exaggerated.
In the 1990s, Shell was forced to stop operating in Ogoniland after mass protests against the lack of investment and environmental damage culminated in a military crackdown.
Then, a special tribunal found Nigerian writer-activist Ken Saro-Wiwa guilty of complicity in the murders of four Ogoni chiefs. The government executed him and other activists in a move widely condemned internationally.
Shell last year paid $15.5 million in an out-of-court settlement in a civil case brought by members of Saro-Wiwa's family and others.
Now Shell has accepted responsibility for two oil spills in 2008 and 2009.
The Bodo fishing community sued Shell in Britain alleging that oil spills in 2008 and 2009 had destroyed the environment and ruined the livelihoods of 69,000 people.
Martyn Day, who represents the Bodo people, said cases like this one languished in the Nigerian court system for years, if not decades. That's why the claim was brought in London, Day said.
"In the end there was no resolution, no compensation and there was no cleanup," Day said.
He said Shell could have to pay out hundreds of millions of dollars in settlements.
The U.N. assessment team examined more than 200 locations over a 14-month period for the assessment.
Among its findings:
-- Some areas that appear unaffected at the surface are in reality severely contaminated underground.
-- In at least 10 Ogoni communities contaminated water has seriously threatened public health.
-- In one community, at Nisisioken Ogale, families are drinking water from wells contaminated with benzene, a carcinogen, at levels over 900 times above World Health Organization guidelines.
Individual contaminated land areas in Ogoniland can be cleaned up within five years, the report said. But heavily affected mangroves and swamplands will take up to 30 years.
Before any clean-up can begin, the report said, ongoing contamination has to end.
CNN's Christian Purefoy contributed to this report.