Pirates have stormed an oil supply vessel off southern Nigeria and kidnapped the ship\'s captain and chief engineer, both US citizens, a private security firm which tracks the region told AFP Thursday. \"The US-flagged C-Retriever was attacked off Brass, Nigeria,\" in southern Bayelsa state, said Richard Fylon of AKE, a London headquartered private security firm, referring to the Wednesday attack. \"Both the chief engineer and the captain were kidnapped,\" he added, saying, \"they are both American.\"