Nigeria's government and oil giant Shell on Friday came under heavy pressure following the release of a landmark U.N. report detailing oil pollution that may require the world's biggest ever clean-up.

The report set out scientific evidence for the first time of devastating pollution in Ogoniland, part of the country's main oil-producing Niger Delta region where Shell and the state petroleum company have operated.

“UNEP believes that oil contamination in Ogoniland has created an environmental crisis of unprecedented proportions,” Joseph Alcamo, U.N. Environment Programme chief scientist, told journalists in London.

Anglo-Dutch Shell was forced to pull out of Ogoniland amid unrest in 1993, though pipelines for its Nigerian joint venture, which includes the state oil company, and other facilities remain there.

The UNEP report, which details urgent health risks such as badly contaminated drinking water, led some to raise the possibility of lawsuits that could now be brought against Shell or others with scientific evidence to back them.

Shell faced criticism from UNEP, which said “control and maintenance of oil field infrastructure in Ogoniland has been and remains inadequate: the Shell Petroleum Development Company's own procedures have not been applied, creating public health and safety issues.”

UNEP also called for the oil industry and the Nigerian government to contribute $1 billion to a clean-up fund for the region, adding that restoration could take up to 30 years.

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