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LAGOS, Nigeria -- Royal Dutch Shell PLC must clean up three-year-old oil spills that have destroyed the livelihoods of tens of thousands of farmers and fishermen in Nigeria's oil-rich southern delta, Amnesty International said Thursday.

spills caused by operational issues in 2008, but said that the vast majority of those harming the area are caused by oil thieves and saboteurs.

Nigeria -- one of the top crude oil suppliers to the United States -- requires companies to promptly clean up oil spills but the policy is not enforced.

Earlier this year, the Bodo community in the Niger Delta took their claim for damages to a UK court following two major oil spills in 2008. The U.N. has recommended that Shell pay an initial $1 billion to begin a 30-year cleanup in the oil-stained region.

Shell's "prolonged failure" to clean up continues to have "catastrophic consequences," say Amnesty International and the Centre for Environment, Human Rights and Development.

"It is time this multibillion dollar company owns up, cleans up and pays up," said Aster van Kregten, Amnesty International researcher for Nigeria.

Shell recently started posting photographs and reports on a website from every oil spill it has investigated this year in a bid to redress the image of a company long demonized by environmentalists in Africa's most populous nation.

"If Amnesty really wanted to make a difference in the Niger Delta, it would join with us in calling for more action to address this criminal activity, which is responsible for the majority of spills," the company said in a statement.

The Amnesty report comes two days after a fresh spill in Nigeria's oil-rich southern delta was followed by a fire on the Shell's Nigerian subsidiary's Okordia-Rumuekpe trunkline in Bayelsa state. The company says it is investigating the source of the spill.

The Anglo-Dutch company was forced out of Ogoniland by residents who accused it of a hand in the 1995 hanging of activist Ken Saro-Wiwa, but pipelines and flowstations operated by a subsidiary of Royal Dutch Shell PLC and the state-run Nigerian National Petroleum Corp. still run through villages and fields.