Pirates attacked an oil supply vessel off the Nigerian coast and kidnapped the captain and chief engineer, both U.S. citizens, a U.S. defence official and security sources said Thursday.

Pirate attacks off Nigeria's coast have increased 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.

"We are seeking additional information so we can contribute to the safe resolution of the situation," White House spokesman Jay Carney said. "Our principal concern now is the safe return of two American citizens."

The captain and an engineer were taken from the offshore supply vessel during an attack Wednesday in international waters off West Africa's Gulf of Guinea, said a U.S. defence official, who spoke on condition of anonymity because the official wasn't authorized to publicly discuss the information.

The two were kidnapped from the U.S.-flagged C-Retriever, a 67-metre ship owned by U.S. marine transport group Edison Chouest Offshore, the official said. The attack and kidnapping were confirmed by UK-based security firm AKE as well as other security sources. The vessel and 11 other members of the crew were released, and the two hostages are believed to have been taken to shore in Nigeria, the official said.

Nigerian navy Capt. Kabir Aliyu also confirmed the attack and said the Nigerian navy is involved in a search for the crew members. Aliyu did not respond to a question asking whether the Nigerian navy was working with U.S. Marines who are in the area for training.

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In April, gendarmes based in the Ivory Coast port of Abidjan - one of west Africa's busiest ports - began a mission to hold the front line against piracy in the Gulf of Guinea following an expansion of Nigerian gangs being blamed for a rise in maritime crime in the region. (Thierry Gouegnon/Reuters)

A spokesman for the Louisiana-based transport company did not return multiple phone calls and emails seeking comment, the Associated Press said. It was unclear whether a ransom demand or any other demands had been made.

U.S. State Department spokeswoman Marie Harf said the U.S. has been concerned by "a disturbing increase" in maritime crime, including piracy, in the area.

Asked what the U.S. would do about the incident and about the long-term piracy issue in Nigeria, she said, "At this point, we're still looking into it. We are concerned by this increase… and will continue to work with states on the Gulf of Guinea to help them respond effectively to maritime crime in these waters."

Gulf of Guinea new hot spot

U.S. Navy Secretary Ray Mabus 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.

In July, the international agency that monitors piracy said well-armed pirates are widening their area of operations and using new strategies in the oil-rich Gulf of Guinea. The London-based International Maritime Bureau said that while piracy is down in the rest of the world, the Gulf of Guinea has overtaken Somalia as the world's new hot spot, according to figures for the first six months of the year.

There are calls for a coalition of naval forces to patrol the strategic area, but naval forces from other countries in the Gulf of Guinea have said they don't have the same capacity as Nigeria to fight piracy.

Piracy spreading

"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.

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 a target for criminals and jacks up 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. 

With files from Reuters