The Pentagon said 13 fast boats from Iran’s Islamic Revolutionary Guard Corps Navy (IRGCN) came within 140m (459ft) of US vessels on Monday.
The Iranian vessels left after 30 warning shots were fired, the Pentagon said.
This is the second such incident in the region in two weeks.
Pentagon spokesman John Kirby accused the Iranian boats of “acting very aggressively” near the US military vessels, which were escorting USS Georgia, a guided-missile submarine.
“It’s unsafe, it’s unprofessional. It’s the kind of activity that can lead to somebody getting hurt, and can lead to a real miscalculation there in the region,” he said. “That doesn’t serve anybody’s interests.”
The Strait of Hormuz, a narrow channel in the Gulf region, is one of the world’s most important shipping routes.
It has been at the centre of rising tensions between Iran and Western countries – including the US – in recent years.
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Last month, the US Navy said one of its ships fired warning shots in response to the close approach of Iranian vessels in the northern part of the Gulf.
Monday’s incident comes at a delicate time in a diplomatic process to revive an international deal to prevent Iran from developing nuclear weapons.