Nigeria's Ogoniland region could take 30 years to fully recover from oil spills, a UN report says.
The long-awaited UN study says environment restoration could prove to be the world's "most wide-ranging and long-term oil clean-up" ever taken.
The report found that pollution seriously threatened public health in at least 10 communities in the region.
Oil giant Shell has accepted liability for two spills which devastated communities in 2008 and 2009.
One community said it would seek hundreds of millions of dollars in compensation. Shell said it would settle the case under Nigerian law.
The UN report, which follows a two-year investigation, has already proved controversial in part because it was funded by Shell.
Nigeria is one of the world's major oil producers.
'Not to blame'
Earlier, the UN Environment Programme (Unep) presented its findings to Nigeria's President Goodluck Jonathan.
"This is not an assessment designed to blame any particular stakeholder operating in Ogoniland," Unep spokesman Nick Nuttall told the BBC's Network Africa programme earlier on Thursday.
"What we are indeed really seriously hoping is that this might actually close the chapter in what has often been a sad, tense and sometimes violent story, going back several decades.
"We are hoping that this might build some sense of co-operation between all the various players in this part of the world."
He also stressed that Shell's admission of liability for two spills had nothing to do with the Unep report.
Martin Day, a lawyer for the Bodo people of Ogoniland, on Wednesday told the BBC that much of the area had been devastated by oil pollution, meaning the local fishing community could no longer sustain their traditional livelihoods.
"As a result of this spill, virtually all of them can no longer fish," he said. "They're left, many of them, in the most poverty-stricken way."
Mr Day described the spill as one of the world's worst but said it had been ignored until his firm threatened to take Shell to court in the UK.
This, he said, could set a legal precedent for other communities whose lives have been hit by Western firms.
The environmental problems caused by the oil industry in Ogoniland were highlighted by the writer Ken Saro-Wiwa, who was executed in 1995 by Nigeria's military government, sparking international indignation.
The campaign forced Shell to stop pumping oil out of Ogoniland but it continues to operate pipelines in the region and spillages have continued.
A previous Unep report which blamed sabotage and theft by local people for 90% of the oil spills caused outrage among local activists.
Ahead of the report, Mr Saro-Wiwa's Movement for the Survival of the Ogoni People (Mosop) condemned it, saying it had not been adequately consulted.