LAGOS, Nigeria, May 09, SPA -- Gunmen seized four American workers as violence that has engulfed Nigeria's southern petroleum-producing region escalated, the Associated Press quoted oil industry officials as saying on Wednesday. The attackers, carrying assault rifles androcket-propelled grenades, stormed a vessel carrying theworkers in the southern Niger Delta minutes before midnight on Tuesday. Chevron said that the four kidnapped expatriate workers were contractors with American citizenship, Dow Jones reported from London. The U.S. embassy in Nigeria could not confirm the information.--SPA www.spa.gov.sa/448152