(CNN) -- The Nigerian navy has yet to find two U.S. citizens kidnapped from an oilfield supply ship attacked this week in the piracy-plagued Gulf of Guinea off Africa's western coast, a spokesman said Friday.
The Americans -- a captain and a chief engineer -- were kidnapped off the 221-foot oilfield supply ship C-Retriever. They remained missing amid a search-and-rescue effort led by the Nigerian navy, spokesman Kabir Aliyu, told CNN on Friday.
The ship itself was in a port on Friday, said a U.S. official who didn't disclose the port's location. A second U.S. official said 11 people had been aboard the ship before it was attacked.
Details about the whereabouts and conditions of the other nine have not been released.
Aliyu vowed that Nigerian forces would "bring the criminals to justice."
The Nigerian navy said Thursday that it would try to rescue the kidnapped captain and chief engineer. But it was unclear whether anyone other than their captors know where the two were taken after their seizure Wednesday.
It is not uncommon for pirates to seize merchant crewmen for ransom, but it was unclear whether that was the scenario playing out Friday.
State Department spokeswoman Marie Harf on Thursday said the administration was "seeking additional information about the incident so that we may contribute to safely resolving the situation."
"Obviously our concern at this point is for the safe return of the two U.S. citizens," she said.
Louisiana-based Edison Chouest Offshore, which owns the vessel, did not immediately respond to requests for comment.
The gulf, a vast expanse of water increasingly plied by pirates, is extremely dangerous, said Don Marcus, president of the International Organization of Masters, Mates & Pilots.
"They're out for anything they can get their hands on," Marcus said of the pirates. "Cargo theft is a big part of it, be it machinery, supplies, be it the actual fuel, be it prisoners that they take ashore and hold for ransom, kidnapping essentially. They're looking for money."
And slow-moving vessels that service oil platforms are more vulnerable to attacks than cargo ships traveling off the coast of Somalia, another area that's drawn attention for maritime piracy, he said.
The Gulf of Guinea produces about 5.4 million barrels of oil a day, according to London-based think tank Chatham House. And about 30% of U.S. oil imports flow through the region, according to International Crisis Group.
The oil-rich area off the coast of West Africa 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, Chatham House reported in March.
The think tank reported 62 pirate attacks in the gulf in 2012, up from 39 in 2010. It says that's partially because Western navies have cracked down on piracy off the coast of Somalia, on the other side of the continent.
U.S. Marines are 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 that were designed to strengthen maritime security, according to the U.S. Navy.
It was unclear, however, if Western forces planned any response to Wednesday's attack.
CNN's Vladimir Duthiers contributed to this report.