A U.S. Navy vessel was forced to fire warning shots at an armed Iranian patrol boat in the Persian Gulf on Tuesday, according to Defense officials.

At approximately 3:00AM local time, the Iranian boat, believed to be a part of Iran’s Revolutionary Guard, set course to intercept the USS Thunderbolt, a Cyclone class patrol ship currently conducting exercises in the region along with a number of other American vessels, including the Ticonderoga-class guided missile cruiser USS Vella Gulf.  The Iranian boat came to within 150 yards of the Thunderbolt, despite a number of warnings issued via radio, firing flares, and blowing the vessel’s whistle as a warning.  The type of boat employed by the Iranians has not yet been disclosed.

Blowing the ship’s whistle in five short blasts is considered an internationally recognized communications signal for danger.

At that point, the decision was made to fire warning shots into the water ahead of the Iranian boat.