A US Coast Guard ship fired warning shots at Iranian boats that came close to American naval vessels in the Strait of Hormuz, the Pentagon says.
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."