A US military ship fired warning shots after three Iranian navy vessels came close to it and another American patrol boat in the Gulf, the United States military said on Tuesday.
Such incidents have occurred occasionally over the past five years, although there has been a relative lull over the past year.
While officials said it was too early to discern the exact motive of the Iranian forces, in the past such incidents were carried out by local commanders rather than directed by senior Iranian leaders.
"The US crews issued multiple warnings via bridge-to-bridge radio and loud-hailer devices, but the IRGCN [Iran's Islamic Revolutionary Guard Corps Navy] vessels continued their close-range manoeuvres," the military said.
"The crew of Firebolt then fired warning shots, and the IRGCN vessels moved away to a safe distance from the US vessels," it said.
The closest the Iranian fast inshore attack craft came to the American ships was 62 metres during the incident, which occurred on Monday in international waters in the northern Gulf.
The incident came as world powers and Iran seek to speed up efforts to bring Washington and Tehran back into compliance with the 2015 nuclear accord, and the US has reassured its Gulf Arab allies on the status of the talks.
Earlier on Tuesday, the head of US forces in the Middle East said the country's military was careful not to let incidents with the IRGCN turn into a cycle of provocation, although he did not mention Monday's face-off.
"The activities we typically see from the IRGC Navy are not necessarily activities that are directed by the supreme leader or from the Iranian state, rather irresponsible actions by local commanders on the scene," said Gen Kenneth McKenzie, head of US Central Command.
A US official said the area of the Gulf where the incident occurred had a history of IRGCN vessels harassing fishing boats and was not overly concerning.
"The US is not an aggressor … our forces are trained, however, to conduct effective defensive measures when necessary," the US military said.