A US Coast Guard vessel in the Strait of Hormuz fired warning shots at Iranian boats after they ignored warnings to stay at a safe distance, the US has said.
In a briefing at the Pentagon on Monday, US Press Secretary John F Kirby said 13 "fast boats" from the Islamic Revolutionary Guard Corps Navy (IRGCN) approached six US Navy vessels that were escorting the guided-missile submarine USS Georgia.
"This group of fast-attack boats approached the US formation at high speed, closing in as close as 150 yards," he said.
"After following all the appropriate and established procedures involving ships: horn blast, bridge-to-bridge radio transmissions and other ways of communicating, the US Coast Guard Cutter Maui... fired approximately 30 warning shots from a 50 caliber machine-gun.
"After the second round of warning shots, the 13 fast-attack craft from the IRGCN broke contact," he added.
The US Press Secretary said the US Coast Guard cutter had fired the warning shots while the IRGCN boats were at the 300-yard mark and then once more when they had reached the 150-yard mark.
He said it was not clear who was directing the Iranian vessels but added: "Harassment by the IRGC Navy is not a new phenomenon.
"It is something that all our commanding officers and crews of our vessels are trained for when serving in the Central Command area of responsibility, particularly in and around the Gulf.
"Our commanding officers and crew of our ships — they have the right of self-defense and they know how to use that right.
"They have the means at their disposal to defend their ships and their crews and they also, as I think we've seen now in this second incident, are very stringent about following the proper procedures for providing warnings: verbally first, and then if need be, through the use of warning shots to try to change or mitigate the Iranian behaviour."
He added: "This 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."
The IRGCN has rejected the US Navy's claim that its boats sparked the incident.
The Guard's website, sepahnews.com, published a statement on Tuesday saying the Americans were guilty of using "false narratives and unprofessional behaviour" and should more strictly "abide by international regulations".
Watch: American and Iranian ships were involved in an incident in the Persian Gulf last month – the US called the Iranian manoeuvre "unsafe".
Specifically, the statement said the Revolutionary Guard's navy warned the US vessels to stop their "provocative and aimless shooting".
The Strait of Hormuz is one of the most important waterways in the world and has been an area of heightened tension.
It was the second time in two weeks that a US ship opened fire to warn vessels of Iran's paramilitary Revolutionary Guard.
On 26 April, an American warship fired warning shots when vessels of Iran's Revolutionary Guard came too close to a patrol in the Persian Gulf.
That was the first such shooting in nearly four years.
American and Iranian ships were also involved in a separate incident in the Persian Gulf earlier in April.
Footage released by the US Navy from 2 April showed what it said was a ship, commanded by the Revolutionary Guard, cutting in front of the US Coast Guard Cutter Monomoy, causing the vessel to stop quickly.
The US military called the manoeuvre "unsafe".
The incidents come as Iran negotiates with world powers to return to the deal that then-US President Donald Trump announced he was pulling out of in 2018.