Five boats thought to be Iranian tried to stop a British oil tanker in the Strait of Hormuz, according to US officials.
The boats withdrew after a warning from a British warship operating in the area.
The incident comes as a Royal Navy warship was said to be offering extra protection to a second tanker in the Gulf following a threat from Iran to seize a British vessel.
Sky News understands HMS Montrose, a Type 23 frigate, has been travelling with a tanker, British Heritage, according to a ship tracking website and a defence source.
Tensions have been rising since Royal Marines helped seize an Iranian-flagged tanker in Gibraltar which was allegedly carrying oil to Syria, in breach of European Union sanctions.
The capture of the Grace 1 tanker prompted Iranian President Hassan Rouhani to warn the UK of "consequences".
"You (Britain) are the initiator of insecurity and you will realise the consequences later," Rouhani said after a cabinet meeting on Wednesday, according to the semi-official news agency Tasnim.