A US warship has fired about 30 warning shots after 13 vessels from Iran’s Islamic Revolutionary Guard Corps Navy (IRGCN) came close to it and other US navy vessels in the Strait of Hormuz, the Pentagon says.

This is the second time within the last month that US military vessels have had to fire warning shots because of what they said was unsafe behaviour by Iranian vessels in the region, after a relative lull in such interactions over the past year.

Pentagon spokesman John Kirby said the warning shots were fired after the Iranian fast boats came as close as 140 metres of six US military vessels, including the USS Monterey, that were escorting the guided-missile submarine Georgia.

The US Coast Guard cutter Maui fired the warning shots from a .50-calibre machine gun before the Iranian vessels left, Mr Kirby said.

“It’s significant… and they were acting very aggressively,” he said, adding that the number of Iranian vessels was more than in the recent past.

In April, a US military ship fired warning shots after three vessels from IRGCN came close to it and another US patrol boat in the Gulf.

The latest incident comes as world powers and Iran seek to speed up efforts to bring the US and Iran back into compliance with the 2015 nuclear accord.

US officials returned to Vienna last week for a fourth round of indirect talks with Iran on how to resume compliance with the deal, which former US president Donald Trump abandoned in 2018, prompting Iran to begin violating its terms about a year later.

-AAP