* Nigeria says its Navy to attempt rescue
* U.S. calls incident an act of piracy, not 'terrorism'
* U.S. concern about piracy off West African coast grows
By Joe Brock and Andrea Shalal-Esa
ABUJA/WASHINGTON, Oct 24 (Reuters) - Pirates attacked an oil supply vessel off the Nigerian coast and kidnapped the captain and chief engineer, both U.S. citizens, American officials said on Thursday as the Nigerian military ordered its Navy to rescue the men.
"We believe this was an act of piracy," U.S. State Department spokeswoman Marie Harf said, adding that U.S. officials were closely monitoring the situation and seeking more information.
"At this point, we do not have information that would indicate this was an act of terrorism," Harf told reporters in a briefing. "Obviously, our concern at this point is for the safe return of the two U.S. citizens."
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 metre) vessel owned by U.S. marine transport group Edison Chouest Offshore, was attacked early Wednesday, UK-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.
However, representatives for the Nigerian Navy said they were aware of the incident and taking action. "We have directed the central Naval Command to see to their rescue. So our men are on top of the situation," spokesman Kabiru Aliyu told Reuters.
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.
The White House said on Thursday it is increasingly concerned about the rise in piracy off the coast of West Africa.
"More broadly, we are concerned by the disturbing increase in the incidence of maritime crime, including incidents of piracy off the coast of West Africa, specifically in the Gulf of Guinea," White House spokesman Jay Carney told reporters at a briefing.
Navy Secretary Ray Mabus has 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.
"The piracy threat is spreading even further through the waters of West Africa, and the attacks have been mounting, even as global rates of reported piracy are at their lowest since 2006," said Michael Frodl of U.S.-based consultancy C-Level Maritime Risks.
CONTRAST TO HORN OF AFRICA
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.
This makes them targets for criminals and raises insurance costs. Kidnapped sailors and oil workers taken in Nigerian waters are usually released after a ransom is paid.
In a separate incident, three Nigerian soldiers were killed on Tuesday when armed robbers attacked a vessel carrying construction workers in the creeks of oil-producing Rivers state, the army said on Thursday.
Piracy has regained attention in the U.S. recently since the release of a movie earlier this month chronicling an April 2009 hijacking of a U.S. ship by Somali attackers.
The incident involved a cargo ship seized off the Horn of Africa later rescued by the U.S. Navy, which sent two ships and Navy SEALS to intervene.