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LAGOS, June 8 (Reuters) - Britain advised all its nationals to leave three core states in Nigeria's oil-producing Niger Delta because of rising insecurity, the government said on its Web site on Friday.

The security situation in Bayelsa, Delta and Rivers states, which are home to the bulk of Nigerian oil production and also the Nigerian unit of Royal Dutch Shell <RDSa.L>, made them unsafe for British nationals, the Foreign Office said.

"We therefore advise British nationals to leave. If you stay, you do so at your own risk and should take professional security advice. The level of consular assistance we can provide is limited," it said on its Web site.

Since January 2006, more than 180 foreign nationals have been kidnapped in the Niger Delta, including 31 British nationals, the Web site said.

Militants fighting for more autonomy in the delta began a wave of attacks on oil facilities and kidnappings early last year, but the line between militancy and crime is blurred and diplomats say the region is slipping into anarchy.

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