A British warship has fended off an attempt by Iranian vessels to seize a BP tanker.

CNN cited two US officials as saying three Iranian Revolutionary Guards boats approached the 158,000-dwt British Heritage (built 2017) in the Middle East Gulf in an apparent retaliatory response to the UK's detention of a VLCC in Gibraltar for alleged sanctions breaches.

The BP ship was crossing the Strait of Hormuz when the incident happened, but it was accompanied by the warship HMS Montrose.

The Iranians ordered the tanker to stop in nearby Iranian territorial waters, CNN reported.

A US aircraft flying overhead filmed the incident, it added.

But the boats withdrew after the warship trained its guns on them and ordered them to move away.

"Contrary to international law"

"It was harassment and an attempt to interfere with the passage," a US official told Reuters.

The UK government said HMS Montrose had positioned itself between the three Iranian boats.

A spokesman told the BBC: "Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz.

"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region."

The Islamic Revolution Guards Corps' Navy, through the Guards-linked Fars news agency, tweeted a denial of the seizure attempt.

Iran's foreign minister Javad Zarif called the British allegations “worthless”.

He told Fars: “Apparently the British tanker has passed. What [the British] have said themselves and the claims that have been made are for creating tension and these claims have no value.”

On Wednesday, Iranian president Hassan Rouhani had called the UK "scared" and "hopeless" for using naval vessels to protect its shipping.

"You, Britain, are the initiator of insecurity and you will realise the consequences later," he added.

HMS Montrose had shadowed MOL's UK-flagged 302,000-dwt VLCC Pacific Voyager (built 2009) earlier in the week.

British Heritage had reportedly been sheltering in the Middle East Gulf due to an Iranian threat against UK shipping.

AIS data from Tuesday had showed it awaiting orders, with no destination listed.

BP is not commenting on the situation.

Bloomberg claimed the BP ship was heading to load crude at Iraq’s Basrah oil terminal when it turned around on 6 July.

The UK-flagged tanker had been chartered by Shell to carry the oil to north-west Europe, brokers said.

The fixture has been cancelled, the report added.

Last week in Gibraltar, the 301,000-dwt Ukrainian-owned Grace 1 (built 1997) was alleged to have been heading for Syria with Iranian crude, in breach of EU sanctions. It was boarded by British marines.