By Jim Miklaszewski, Courtney Kube, F. Brinley Bruton and Jason Cumming, NBC News
Two Americans were kidnapped by pirates after their ship was attacked off Nigeria's coast, U.S. officials said Thursday.
The U.S.-flagged oil supply vessel C-Retriever was targeted in the Gulf of Guinea early Wednesday, Reuters reported.
Maritime news website gCaptain reported that the ship's captain and its chief engineer had been abducted.
U.S. officials said the working assumption was that the pair had been kidnapped for ransom.
The seized vessel is owned by Louisiana-based Edison Chouest Offshore, according to Reuters. The company was not immediately available for comment.
Sources told NBC News that there were no U.S. warships in the region and no immediate plans for a hostage rescue attempt. However, there is a contingent of U.S. Marines aboard a Dutch warship in the area as part of a military exchange program.
We are closely monitoring reports that two U.S. citizens have been kidnapped from a U.S.-flagged vessel, the C-Retriever, in the Gulf of Guinea,” a State Department official told NBC News. “We are seeking additional information about the incident.”
In April 2009, U.S. Navy SEAL snipers killed three Somali pirates as they rescued American cargo ship Capt. Richard Phillips, who had offered himself as a hostage to save his crew.
The high-seas hijacking has been turned into a film starring Tom Hanks. "Captain Phillips" earned more than $52 million during its first two weeks in cinemas.
In an interview with TODAY's Matt Lauer on Oct. 11, Phillips said the waters off Nigeria were now "worse than Somalia."
"If you're gonna be in the merchant marines, you’ll have to deal with piracy," Phillips added. "Firemen go into a burning house. Police have to deal with violent situations. Merchant marines have to deal with piracy."
Rory Lamrock, an analyst specializing in maritime security with U.K.-based risk-management firm AKE, said there had been "an increase in the severity of attacks in the Gulf of Guinea" over the past two years.
In August, Nigeria's navy killed 12 pirates as they tried to flee from a fuel tanker they had hijacked.
Earlier this month, the International Maritime Bureau reported that pirate attacks off Nigeria's coast had jumped by a third this year -- with 29 attacks on vessels recorded in the first nine months of 2013, up from 21 in the same period last year.
"Pirates, often heavily armed and violent, are targeting vessels and their crews along the [Nigerian] coast, rivers, anchorages, ports and surrounding waters," the IMB said. "In many cases, they ransack the vessels and steal the cargo."
The IMB said in the first nine months of 2013 the Gulf of Guinea accounted for all crew kidnappings worldwide, 32 of them off Nigeria, and two off Togo. In such incidents, sailors are taken ashore and usually held for ransom.
In a separate report, Denmark-based security firm Risk Intelligence earlier this month estimated117,000 tons of oil products worth around $100 million had been stolen by pirate gangs in the Gulf of Guinea since 2010.
“Attacks by pirates off the coast of Nigeria in the Gulf of Guinea have increased substantially in recent years,” according to a June 2013 travel warning from the State Department. “Armed gangs have boarded both commercial and private vessels to rob travelers. The Nigerian Navy has limited capacity to respond to criminal acts at sea.”
Alistair Galloway, owner of private security provider Endeavour Maritime, said the discovery of new oil reserves in West African countries including Cameroon and Liberia was driving up shipping traffic in the area.
"This oil boom is attracting a far richer shipping environment and therefore more higher-value targets and that is the biggest threat to the region's maritime security," he said.
Galloway cited differences between West African pirates and their better-known Somalia-based counterparts.
"In Somalia, piracy was really out-of-work fisherman looking to improve their lives, but piracy is West Africa is really part of a bigger criminal system, networks embedded in the nations," he said. "The networks are in place and it’s been easy for them to attack."
Johan Potgieter, senior researcher at South Africa's Institute for Security Studies (ISS), said that pirates tended to feel "disenfranchised."
He added: "They feel that if the government won’t share its wealth equally, they will take it for themselves by other means."
NBC News' Henry Austin, Marc Smith, Becky Bratu, Alexander Smith, Catherine Chomiak and Matthew DeLuca contributed to this report. Reuters also contributed.