US and UK sailors came to the aid of a ship in the Strait of Hormuz after Iran's Revolutionary Guard "harassed" the vessel, the US Navy said.
Three fast-attack Guard vessels with armed troops on board approached the merchant ship on Sunday afternoon, a spokesperson said.
The US Navy offered black-and-white images which came from a US Navy Boeing P-8 Poseidon overhead, which showed three small ships close to the commercial vessel.
The US Navy's guided-missile destroyer USS McFaul and the Royal Navy's frigate HMS Lancaster responded to the incident, with the Lancaster launching a helicopter.
"The situation de-escalated approximately an hour later when the merchant vessel confirmed the fast-attack craft departed the scene," the Navy said. "The merchant ship continued transiting the Strait of Hormuz without further incident."
The Strait of Hormuz, the narrow mouth of the Persian Gulf, sees 20% of the world's oil pass through it.
While the Navy did not identify the vessel involved, ship-tracking data from MarineTraffic.com analysed by The Associated Press showed the Marshall Islands-flagged bulk carrier Venture erratically changed course as it travelled through the strait at the time of the incident.
Its location also matched information about the incident given by the United Kingdom Maritime Trade Operations, a British military operation overseeing traffic in the region.
The ship's registered manager, Trust Bulkers of Athens, Greece, did not immediately respond to a request for comment.
There have been a series of maritime incidents involving Iran following the US unilaterally withdrawing from Tehran's nuclear deal with world powers in 2018.
The suspected American seizure of the Suez Rajan, a tanker linked to a US private equity firm believed to have been carrying sanctioned Iranian crude oil off Singapore, likely sparked Tehran to recently take the Marshall Islands-flagged tanker Advantage Sweet. That ship carried Kuwaiti crude oil for energy firm Chevron. of San Ramon, California.
While authorities have not acknowledged the Suez Rajan's seizure, the vessel is now off the coast of Galveston, Texas, according to ship-tracking data analysed by the AP.
Meanwhile, Iran separately seized the Niovi, a Panama-flagged tanker, as it left a dry dock in Dubai, United Arab Emirates, bound for Fujairah on the UAE's eastern coast.
While not carrying any cargo, data from S&P Global Market Intelligence seen by the AP showed the Niovi in July 2020 received oil from a ship known then as the Oman Pride.
In August 2021, the US Treasury sanctioned the Oman Pride and others associated with the vessel over it being "involved in an international oil smuggling network" that supported the Quds Force, the expeditionary unit of the Guard that operates across the Middle East.
Purported emails published online by Wikiran, a website that solicits leaked documents from the Islamic Republic, suggest that cargo carried by the Niovi was sold on to firms in China without permission.
Satellite images analysed by the AP show those two vessels anchored off Bandar Abbas, Iran.