The incident in international waters occurred Tuesday when the Coast Guard patrol vessel Monomoy sent a crew in an inflatable boat to "query the Iranian dhow" and "the dhow's crew trained and prepared to fire a .50 caliber machine gun on their small boat as it approached," according to a statement by the Navy Fifth Fleet, stationed in Bahrain.
The aiming of the machine gun was "demonstrated hostile intent," the statement said, and the Coast Guard vessel fired one round "in self-defense."
It is not known if the dhow was struck by the shot. It left the area and did not communicate with the Monomoy, which earlier determined it was an Iranian vessel.
No U.S. personnel were injured. The U.S. Coast Guard is on patrol in the Persian Gulf with the Navy's Fifth Fleet.