Pirates have attacked an oil supply vessel off the Nigerian coast and kidnapped the captain and chief engineer, both US citizens.

The US-flagged C-Retriever, owned by marine transport group Edison Chouest Offshore, was attacked in the early hours of yesterday, UK-based security firm AKE and two security sources said.

The company was not immediately available for comment.

A US defence official said the State Department and FBI were leading the US response to the incident.

A second defence official said the US Marine Corps has a small training unit in the region, but it was not clear if it would get involved.

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.

US Navy Secretary Ray Mabus called the gulf a potential "hotspot" after a visit to four countries surrounding the gulf in August.

He said the navy was working closely with Gabon, Senegal, Sao Tome and Ghana to help fight an increase in illegal trafficking of drugs, people and arms.

Unlike the dangerous waters off Somalia and the Horn of Africa on the east coast of Africa, through which ships now speed with armed guards on board, many vessels have to anchor to do business off west African countries, with little protection.