ABUJA (AFP) - 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. 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. 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.