(CNN) -- Two mariners believed to be U.S. citizens have been taken from their U.S.-flagged ship following a pirate attack in the West African Gulf of Guinea, a U.S. official confirmed Thursday.

The attack on the oil platform supply vessel C-Retriever happened Wednesday off the coast of Brass, Nigeria. Two crew members -- the captain and chief engineer -- were taken off the ship, the U.S. official said.

Further details on Wednesday's attack, including the well-being of the two crew members or the condition of their ship, were not immediately available.

The C-Retriever is owned by Edison Chouest Offshore, which is based in Louisiana.

The oil-rich gulf has increasingly drawn international attention as a piracy hotspot, with 40 pirate attacks reported in the first nine months of 2013, the International Maritime Bureau reported.

It also has been the site of the only ship crew kidnappings worldwide this year, with 132 crew members taken hostage.

Seven ships have been hijacked, the organization said.

Piracy in the Gulf of Guinea accounted for 30% of the 1,434 reported piracy attacks in African waters between 2003 and 2011 and the pace of attacks has risen since then, London-based think tank Chatham House reported in March.

Chatham House reported 62 pirate attacks in the gulf in 2012, up from 39 in 2010.

The increase is in part due, the think tank said, to aggressive anti-piracy efforts by Western navies off the coast of Somalia, on the east side of the continent.

U.S. Marines are currently in the region aboard a Dutch ship off West Africa. Military forces from the United States, United Kingdom, Spain, the Netherlands and five African nations recently held exercises in the region designed to strengthen maritime security, according to the U.S. Navy.

The gulf produces some 5.4 million barrels of oil a day, according to Chatham House. Some 30% of U.S. oil imports flow through the region, according to International Crisis Group.