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.
