U.S. warships Monday in the Strait of Hormuz fired warning shots at armed Iranian speedboats that had approached to within 150 yards of the American flotilla, U.S. officials said.

Two of the Iranian speedboats, armed with machine guns, broke off from a group of 13 Iranian craft and headed toward the U.S. formation at a high rate of speed, according to U.S. Naval Forces Central Command (NAVCENT).

"After U.S. naval ships repeated verbal and acoustic warning, sounded five blasts of the ship’s horn, and fired warning shots," the two Iranian craft withdrew, according to a statement by NAVCENT.

The formation of six U.S. warships was escorting the guided-missile submarine USS Georgia through the Strait and into the Persian Gulf.

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After "bridge-to-bridge verbal warnings, five acoustic device warnings, and five short blasts of the ship’s horn, the internationally recognized danger signal," the U.S. ships finally began firing warning shots, according to NAVCENT. 

Department of Defense press secretary John Kirby described the Iranian approach as "significant" and "more than we’ve seen in recent past." 

"There was a volley at roughly the 300-yard mark and then another volley of warning shots as they got into one hundred and fifty yards," Kirby said.

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"They were acting very aggressively," Kirby said of the Iranians.

"When you're in the strait, you there are certain limits to your ability to maneuver," he said. "So it's not insignificant that this kind of dangerous, unsafe and unprofessional incident of behavior occurred there.

Lucas Tomlinson contributed to this report.