A United Nations report says decades of oil spills in Nigeria's Ogoniland region may require the world's largest petroleum cleanup that could cost more than $1 billion.

The report, released Thursday by the U.N.'s environmental program, said that drinking water supplies within the oil-rich Niger Delta have been damaged by 50 years of crude oil spills.

The U.N. said drinking water in some areas is contaminated so severely it needs immediate action. The world body suggested the Nigerian government and the oil industry set up an initial $1 billion trust fund for a cleanup effort that could take 30 years.

Activists say oil spills have devastated Ogoniland, a community of mainly farmers and fishermen.

On Wednesday, oil company Royal Dutch Shell accepted responsibility for two Ogoniland spills in 2008 and 2009 and promised to pay compensation to the local Bodo community, which had taken legal action against the company.

Shell was forced to stop operating in Ogoniland in 1993 but its remaining pipelines and other infrastructure continue to cause pollution.

The company argues that sabotage by gangs and criminals cause most the spills, an assertion strongly disputed by activists. Shell says it cleans up spills whatever the cause.

Officials and rights groups say the U.N. report is the most detailed scientific study to examine the effects of oil pollution in the Niger Delta, the heart of Africa's largest oil industry. The wide-ranging, two-year effort was paid for partly by Shell at the request of the Nigerian government, leading to criticism the report would not challenge the oil giant.

The Niger Delta was the scene of many militant attacks between 2005 and 2009 that led to a sharp drop in oil production. The region has returned to relative calm since the government offered an amnesty to militants, and many have surrendered their weapons.

OPEC member Nigeria is one of the top crude oil suppliers to the United States.