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.
Brigadier General Amir Hatami also warned the UK its actions “will not go without a response”.
Tehran claimed the detention was an “illegal interception” and have demanded the tanker’s release.
But Gibraltar officials believe the 330m (1,000ft) tanker could have been carrying up to two million barrels of oil destined for Syria.
Iran has denied this and said the boat was headed “somewhere else”.
Gibraltar’s supreme court ruled last week the tanker could be held for another 14 days, the territory’s attorney general said.