Pirates have detained two US sailors from an oil supply ship off the coast of Nigeria, officials say.
The captain and chief engineer of the US-flagged C-Retriever oil supply vessel were taken on Wednesday by armed men who stormed the boat, they said.
US officials said it was not a terrorist act, but concerned about the rise in piracy near the coastal town of Brass in Nigeria's off West Africa.
Nigerian navy Special Forces patrol the coast against attacks by pirates
In kidnaps in the coastal region of Nigeria, crew members are normally freed unharmed after any oil on board the ships is stolen.
US State Department Deputy Spokeswoman Marie Harf on Wednesday's stated that the incident was being treated as "an act of piracy" rather than terrorism.
The chief engineer and the captain, both American citizens, were kidnapped. The vessel and 11 other members of the crew were reportedly released.
The International Maritime Bureau has recorded more than 40 attacks in the area this year 132 crews taken hostage. The gangs usually target the oil vessels to steal the cargo.
It is estimated that oil product worth approximately $100m (approximately £62m) has been stolen since 2010.
In recent months there have been numerous kidnappings of foreign oil workers and wealthy Nigerians, especially in the oil-producing Niger Delta region.
They have often been held until a ransom payment is made.