Armed pirates have attacked an oil supply vessel off the Nigerian coast and kidnapped the captain and chief engineer, both United States citizens, security sources said on Thursday. The US-flagged, Edison Chouest-owned C-Retriever was attacked in the early hours of Wednesday, UK-based security firm AKE and two security sources said. The company was not immediately available for comment. Piracy in the Gulf of Guinea region, which includes Africa's No. 1 oil producer Nigeria and is a significant source of cocoa and metals for world markets, is increasing costs for shipping firms operating there.