Pirates kidnap two U.S. sailors off Nigerian coast: sources By Joe Brock and Andrea Shalal-Esa ABUJA/WASHINGTON (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. 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, UK-based security firm AKE and two security sources said. 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. 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. 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. 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. Additional reporting by Jonathan Saul in London, Tife Owolabi in Yenagoa, and Susan Heavey, Steve Holland and Roberta Rampton in Washington; Editing by Mark Trevelyan, Doina Chiacu and Cynthia Osterman