LAGOS (AFP) – Gunmen kidnapped three Britons and a Colombian working for Anglo-Dutch oil giant Shell early Tuesday, shooting dead their police escort, police and security sources said. The four were seized around 0600 GMT on a road leading from the southern oil city of Port Harcourt to the Shell-operated Afam power plant, police said. ""They were ambushed by gunmen,"" Rita Abbey, police spokeswoman for Rivers State told AFP. ""They are Shell contractors who were going to Aba in Abia state,"" she said, adding police had launched a search in and around the town situated northeast of Port Harcourt. The gunmen shot dead the police officer escorting the group and wounded their driver. It is the first major kidnapping in southern Nigeria for months, following a lull in the wake of a government amnesty which saw thousands of militants lay down their arms. Armed groups claiming to seek a fairer share of oil revenue for local communities have since 2006 staged attacks on oil installations in the Niger Delta.