Nigerian authorities are continuing their search for two American men kidnapped by pirates off the Gulf of Guinea.
The captain and chief engineer of a U.S.-flagged oil supply vessel were taken on Wednesday during an attack in international waters off the coast of Nigeria.
The 68-meter C-Retriever is owned by the U.S. oil servicing company Edison Chouest Offshore, which has not commented on the situation.
U.S. officials say the vessel and the rest of its crew were released by the pirates. It is not clear if the American men are still at sea or were taken to land.
The Nigerian military has ordered a search and rescue operation. Officials have refused to say whether the U.S. military is also involved.
U.S. defense officials say they are monitoring the situation, noting that the closest U.S. military contingent is a Marine Corps training unit in the Gulf of Guinea.
State Department spokeswoman Marie Harf said the U.S. is concerned about the safe return of its two citizens. She added that U.S. officials are looking into "safely resolving the situation."
Harf also expressed concern at the rising number of pirate attacks in the Gulf of Guinea. The area has seen a recent increase in piracy, even as pirate attacks are down elsewhere in the world.
The International Maritime Bureau recorded 31 pirate attacks in the oil-rich west African region in the first half of 2013.