Decades of oil pollution in Nigeria\'s Ogoniland region may require the world\'s biggest ever cleanup, a UN agency said on Thursday as it released a landmark report on the issue. \"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 UN Environment Programme 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 United Nations Environment Programme. Its report marks the first major attempt to scientifically document the effects of oil pollution in the region of mainly farmers and fishermen, which activists say has been devastated by spills. UNEP called the 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.\" Ogoniland was the native region of Ken Saro-Wiwa, the renowned environmental activist who was executed by the then-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.