Heavily armed gunmen have kidnapped four U.S. oil workers from a barge off the Nigerian coast near Chevron's Escravos crude export terminal, authorities said. The oil workers were taken from a barge off the Nigerian coast on Wednesday (May 9) as violence flared after disputed elections in the world's eighth largest oil exporter. It was the ninth attack on Western oil facilities in as many days in the Niger Delta, home to Africa's biggest oil industry, in the wake of April's general elections which were condemned by observers as fraudulent. Some militants have stepped up attacks to make clear that they are unimpressed by the looming change in government. Others may be taking advantage of a reduction in security after the elections to resume their criminal pursuits, analysts say. No group has claimed responsibility for the latest attack, but an 18-month campaign of attacks by The Movement for the Emancipation of the Niger Delta (MEND) has forced the closure of almost a third of Nigeria's oil capacity. MEND has threatened to launch further attacks before President Olusegun Obasanjo steps down for president-elect Umaru Yar'Adua on May 29. The barge, operated by U.S. contractor Global Industries, was laying pipelines for Chevron at its Okan oilfield. A Chevron spokesman confirmed the abduction and said oil exports were not directly affected. The company ships about 160,000 barrels per day (bpd) from the nearby Escravos terminal. The MEND says it is fighting for local control over the delta's oil wealth but the majority of kidnappings are carried out by criminal gangs seeking cash. The MEND has told oil workers to leave the region and vowed to bring Nigerian exports to a complete halt to press its case for more autonomy from the central government. It also wants the release of two jailed leaders from the delta and $1.5 billion USD compensation to villages for decades of oil spills. Raids on oil installations and abductions of foreign workers have become frequent in the anarchic delta, a maze of mangrove-lined creeks where most people live in poverty despite being home to Africa's largest oil reserves. Last Thursday (May 3) eight foreign workers were briefly abducted from a stationary vessel off the coast of the Niger Delta. They were released within six hours.