A US Navy ship has fired warning shots at an Iranian Revolutionary Guard vessel in the Gulf, US officials say.
It happened at around 03:00 local time (00:00 GMT), as the Iranian ship began to approach the USS Thunderbolt.
A US defence official said the boat came within 450ft (137m), and ignored radio calls and the ship's whistle.
The Iranian Revolutionary Guard is a paramilitary force that answers directly to Iran's Supreme Leader, Ayatollah Ali Khamenei.
"The IRGCN boat was coming in at a high rate of speed. It did not respond to any signals, they did not respond to any bridge-to-bridge calls, they felt there was no choice except to fire the warning shots," the official told AFP news agency.
The Iranian ship stopped after the warning shots, and the Thunderbolt - accompanied by several US Coast Guard vessels - continued on its way.
The incident is the latest in a series of close encounters between ships from the US and Iran.
Years of hostility between the two nations eased slightly last year after Washington lifted sanctions on Tehran as part of a deal to curb its nuclear ambitions.
However, tensions remain over the Syrian war and Iran's ballistic missile programme, and stand-offs at sea have happened.
In January, the USS Mahan destroyer fired three warning shots when Iranian fast-attack vessels approached at high speed in the Strait of Hormuz.
In September 2016, during his presidential campaign, Donald Trump threatened that any Iranian ships hassling the US Navy in the Gulf would be "shot out of the water".