Pirates attacked an oil supply vessel off the Nigerian coast and kidnapped the captain and chief engineer, both United States citizens, an American defense official and security sources said on Thursday.

Reuters reports that 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 come under threat from gangs wanting to snatch cargoes and crews.

The U.S flagged, C-Retriever, a 222-foot (67 meter) vessel owned by U.S. marine transport group Edison Chouest Offshore, was attacked early Wednesday, the United Kingdom-based security firm AKE and two security sources said.

The company was not immediately available for comment.

A U.S. defense official said the State Department and FBI were leading the American response to the incident. A second defense official said the U.S. Marine Corps has a small training unit in the region but it was not clear if it would get involved.

U.S. Navy officials have grown increasingly concerned about piracy and armed robbery in the Gulf of Guinea and are working with local authorities there to strengthen their ability to patrol the region and better share information.

Navy Secretary Ray Mabus called the region a potential “hot spot” after a visit to four countries surrounding the gulf in August. He told Defense News in September 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.