WASHINGTON (AP) — Pirates kidnapped two Americans working on a commercial ship near the coast of Nigeria, U.S. officials said Thursday. And the Nigerian navy ordered its forces to mount a rescue operation.

"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 away from an offshore supply vessel during an attack Wednesday in international waters off the Gulf of Guinea, said a U.S. defense 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 222-foot ship owned by Edison Chouest Offshore of Louisiana, the official said. The vessel and 11 other members of the crew were released and the two hostages were believed taken to shore in Nigeria, the official said.

Nigerian navy Capt. Kabir Aliyu confirmed that there was an attack. He said the Nigerian navy has directed its operational commands and bases "to search (for) and rescue the crew members and the vessel." 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.

A spokesman for the U.S. company did not return multiple phone calls and emails seeking comment. It was unclear whether a ransom demand or any other demands had been made.

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. We've worked and will continue to work with states on the Gulf of Guinea to help them respond effectively to maritime crime in these waters."

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The U.S. defense official said the U.S. Navy was monitoring the situation and that the closest military vessel was a Dutch navy ship in the Gulf of Guinea with 90 U.S. Marines aboard as part of an Africa training mission.

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Associated Press writers Michelle Faul in Lagos, Nigeria; Darlene Superville, Lolita C. Baldor and Deb Riechmann in Washington; and Kevin McGill in New Orleans contributed to this report.