A US Navy destroyer has fired three warning shots at Iranian patrol boats in the Strait of Hormuz, the entrance to the Persian Gulf.
Officials say four fast-attack boats were closing on the USS Mahan at high speed and ignored attempts to make contact.
Article continues belowThe Week
Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.
Sign up for The Week's Free Newsletters
From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.
From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.
Reuters reports the Mahan "fired flares" while a "US Navy helicopter also dropped a smoke float before the warning shots" in an attempt to warn the Iranian vessels away.
Pentagon spokesman Captain Jeff Davis described the actions of the Iranian vessels as "unsafe and unprofessional".
He said: "They were approaching at a high level of speed with weapons manned and disregarding repeated warnings.
"It's somewhat out of character, recently anyway, from what we've seen out of Iran."
Davis said there had been a total of 35 encounters "assessed to be unsafe and unprofessional" in 2016.
"The vast majority of those were in the first half of 2016," he added.
White House press secretary Josh Earnest said: "These types of actions are certainly concerning and certainly risk escalating tensions."
The incident is the latest tense encounter between the two countries in and over waters near Iran in recent months, says CNN. "These brushes have included Iranian rocket launches, drones flying over US vessels and the capture of US sailors," it adds.
The Guardian says the incident "could be seen as Iranian probing amid uncertainty over how quickly and severely Trump will turn away from Obama's policy of engaging diplomatically with Iran".
In September 2016, Trump said any Iranian vessels that harass the US Navy in the Gulf would be "shot out of the water".