ABUJA/LONDON, Oct 24 (Reuters) - Pirates have attacked an oil supply vessel off the Nigerian coast and kidnapped the captain and chief engineer, both U.S. citizens, security sources said on Thursday.

Pirate attacks off Nigeria's coast have jumped by a third this year as ships passing through West Africa's Gulf of Guinea, a major commodities route, have increasingly come under threat from gangs wanting to snatch cargoes and crews.

The U.S.-flagged, C-Retriever, owned by U.S. marine transport group Edison Chouest Offshore, 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. read more