A US Navy vessel fired a warning flare towards an Iranian military speedboat that approached it in the Strait of Hormuz, the US Fifth Fleet said.
The incident Monday involving the patrol ship USS Sirocco comes amid tensions as Western powers and the Islamic republic seek to salvage their tattered 2015 nuclear deal.
"Three vessels from Iran's Islamic Revolutionary Guard Corps Navy interacted in an unsafe and unprofessional manner as US Navy ships transited the Strait of Hormuz," said the US Navy statement.
"One of the vessels approached Sirocco head-on at a dangerously high speed and only altered course after the US patrol coastal ship issued audible warning signals to avoid collision.