WASHINGTON — A US Coast Guard ship fired 30 warning shots as a "large group" of Iranian Islamic Revolutionary Guard Corps Navy fast boats conducted "unsafe and unprofessional maneuvers" near US naval vessels in the Strait of Hormuz on Monday, Pentagon spokesman John Kirby said. Six US Navy vessels were escorting a guided missile submarine, the USS Georgia when they encountered a group of 13 IRGCN "fast attack boats," Kirby told reporters. The boats approached the US Navy vessels "at high speed," coming as close as within 150 yards. After the US tried "all the appropriate and established procedures involving hip blasts, bridge to bridge radio transmission and other ways of communicating," US Coast Guard Cutter Maui fired approximately 30 warning shots, Kirby said. Following the second round of warning shots, the IRGCN fast boats "broke contact." The episode follows a string of recent incidents between Iranian boats and US warships. Last month, Iran"s Navy came within 68 yards of US ships operating in the Persian Gulf. And earlier in April, four IRGC ships, including three fast attack craft and a support vessel, came within 70 yards of two Coast Guard cutters. "Sadly, harassment by the IRGC navy is not a new phenomenon," Kirby acknowledged on Monday. "As we said at the top, it"s unsafe, it"s unprofessional, and this kind of activity is the kind of activity that could lead to somebody getting hurt, and could lead to a real miscalculation there in the region and that doesn"t serve anybody"s interests." During Monday"s encounter, there were two different volleys of shots fired by the US Coast Guard Cutter Maui, Kirby said. One round was fired when the IRGCN boats were within 300 yards of US Navy vessels. The second round was fired when they were within 150 yards. "It"s an international waterway, and of course, when you"re in the strait, there are certain limits to your ability to maneuver, I mean it is a chokepoint in the region, so it"s not insignificant that this kind of dangerous, unsafe and unprofessional behavior occurred there," Kirby added. — CNN