The Royal Navy and US military responded to a distress call from a merchant vessel passing through the Strait of Hormuz after three Iranian fast-attack boats "harassed" it, the US Navy said.
A still image taken from the surveillance video from a US Navy Boeing P-8 Poseidon shows three vessels in close proximity to a commercial ship.
US Navy guided-missile destroyer USS McFaul and Royal Navy Type 23 frigate HMS Lancaster both received the distress call with Lancaster launching a helicopter to provide surveillance for the mission, the US Navy said.
"The internationally flagged merchant vessel made a radio distress call at 4:56pm local time while transiting the narrow strait," the US Navy said in a statement
"The civilian crew reported three fast-attack craft with armed personnel approached and followed the merchant vessel at close distance.
"The situation de-escalated approximately an hour later when the merchant vessel confirmed the fast-attack craft departed the scene," the statement went on to say, and added that "the merchant ship continued transiting the Strait of Hormuz without further incident".
Wildcat helicopters on board HMS Lancaster are armed with air-to-surface anti-ship missiles, Martlet and Sea Venom, which are capable of fending off fast-attack craft.
The Strait of Hormuz is located at the mouth of the Persian Gulf and is crucial to global energy supplies, with about a fifth of all oil traded at sea passing through it.