That fleet of merchant ships owned by United States citizens or corporations and registered under flags of convenience or necessity such as Liberia or Panama. The term is used to emphasize that, while the fleet is not US-flag, it is effectively under U.S. control by virtue of the ship's owners and can be called to serve U.S. interests in time of emergency. U.S.-flag vessels are registered in the United States and are subject to additional U.S. laws and regulations to which foreign-flag vessels are not. They must be owned by U.S. citizens, corporations, or governments and must be crewed mainly by U.S. citizens