Armed Iranian boats are reported to have tried to intercept a British oil tanker in the Strait of Hormuz – before being driven off by a Royal Navy ship.
Media reports quoting US officials said the tanker was asked to stop in Iranian waters close by, but the boats withdrew after a warning from the UK’s frigate.
Iran has been threatening to retaliate for the seizure of one of its tankers near Gibraltar last week.
The Pentagon said it was aware of the reports but the MoD has not commented.
According to the reports, five boats believed to belong to the Iranian Revolutionary Guard Corps approached the British Heritage tanker as it was moving out of the Gulf.
Last week, British Royal Marines helped the authorities in Gibraltar seize an Iranian oil tanker because of evidence it was heading to Syria in breach of EU sanctions.
In response, an Iranian official said a British oil tanker should be seized if its detained ship was not released.
Iran also summoned the British ambassador in Tehran to complain about what it said was a “form of piracy”.
And on Wednesday Iranian President Hassan Rouhani mocked the UK, calling it “scared” and “hopeless” for using Royal Navy warships to shadow a British tanker in the Gulf.
HMS Monrose had shadowed British tanker the Pacific Voyager for some of the way through the Strait of Hormuz, but that journey passed without incident.
This latest row comes at a time of escalating tensions between the US and Iran.
The Trump administration – which has pulled out of an international agreement on Tehran’s nuclear programme – has reinforced punishing sanctions against Iran.
Its European allies, including the UK, have not followed suit.
Nonetheless, the relationship between the UK and Iran has also become increasingly strained, after Britain said the Iranian regime was “almost certainly” responsible for the attacks on two oil tankers in June.
The UK has also been pressing Iran to release British-Iranian mother Nazanin Zaghari-Ratcliffe who was jailed for five years in 2016 after being convicted for spying, which she denies.