Decades of oil pollution in Nigeria's Ogoniland region may require the world's biggest ever clean-up, the UN environmental agency said Thursday as it released a landmark report on the issue.

The UN Environment Programme also called for the oil industry and the Nigerian government to contribute $1 billion to a clean-up fund for the region that activists say has been devastated by pollution.

"The environmental restoration of Ogoniland could prove to be the world's most wide-ranging and long term oil clean-up exercise ever undertaken if contaminated drinking water, land, creeks and important ecosystems such as mangroves are to be brought back to full, productive health," the UNEP said in a statement. [yahoo]

The study of the effects of pollution in Ogoniland, part of the Niger Delta, the country's main oil-producing region, follows a two-year assessment by the UN's environmental agency.

Its report marks the first major attempt to scientifically document the effects of oil pollution in the region of mainly farmers and fishermen. UNEP called the wide-ranging assessment "unprecedented".

The report documents major health risks in the region of Africa's largest oil producer.

"In at least 10 Ogoni communities where drinking water is contaminated with high levels of hydrocarbons, public health is seriously threatened," the UNEP statement said.

"In one community, at Nisisioken Ogale, in western Ogoniland, families are drinking water from wells that is contaminated with benzene -- a known carcinogen -- at levels over 900 times above World Health Organization guidelines.

"The site is close to a Nigerian National Petroleum Company pipeline."

After being presented with the report, Nigerian President Goodluck Jonathan, the first head of state from the Niger Delta, said the government would consider how to move forward.

"The Nigerian government is going to discuss with Shell and other oil companies that have operated in the area and other relevant agencies of government to see how we can handle this report," he said.

Ogoniland was the native region of Ken Saro-Wiwa, the renowned environmental activist who was executed by a Nigerian military government in 1995 after what was widely considered a show trial, drawing global condemnation.

His activism and execution drew the world's attention to Ogoniland.

Anglo-Dutch oil giant Shell, Nigeria's oldest and historically its largest operator, was forced to leave Ogoniland in 1993 following community unrest sparked by poverty and allegations of environmental neglect.

Amnesty International took aim at Shell in its reaction to the report, alleging it has failed to deal with years of oil spills. Shell has said that most spills in the delta have been caused by theft and sabotage.

"Shell must put its hands up, and face the fact that it has to deal with the damage it has caused," said Audrey Gaughran of Amnesty.

"Trying to hide behind the actions of others, when Shell is the most powerful actor on the scene, simply won?t wash."

Shell declined to comment in detail until it had a chance to study the full report.

Amnesty has estimated that, if all types of oil pollution in the vast Niger Delta are added up over the past half-century, it would be "on par with the Exxon Valdez every year over the last 50 years."