London said Thursday (July 11th) that Iranian military vessels tried to "impede the passage" of a UK oil tanker but were warned off by a British warship in a dramatic escalation of tensions with Tehran in the Gulf.
Iran's Islamic Revolutionary Guard Corps (IRGC) denied involvement but also cautioned both the US and Britain that they would "strongly regret" the UK Royal Marines' detention of an Iranian tanker off Gibraltar last week.
Wednesday's incident in the flashpoint Strait of Hormuz occurred after US President Donald Trump announced that sanctions against Iran over its stepped-up nuclear activities would be "increased substantially" soon.
The rapid chain of events further complicates the efforts of Britain and other European allies to salvage a landmark nuclear accord with Iran from which Washington pulled out last year.
CNN initially cited US defence sources as saying that armed Iranian vessels belonging to the IRGC tried to seize the tanker but were driven off by a Royal Navy frigate.
The UK defence ministry said only that the Iranian boats tried to "impede" a commercial vessel called British Heritage. The 274-metre supertanker is owned by the British energy giant BP and can carry a million barrels of oil.
"We are concerned by this action and continue to urge the Iranian authorities to deescalate the situation in the region," a Downing Street spokesman said.
The UK defence ministry said "three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz".
"HMS Montrose was forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away," the ministry statement said.
The IRGC, which the US blames for staging several tanker attacks since May, denied trying to seize or impede the UK tanker.
BP said in a brief statement that its "top priority is the safety and security of our crews and vessels" but made no further comment.
The episode adds further fuel to a volatile mix of brinkmanship in the region and follows an incident in which an Iranian tanker was halted by police and customs agencies in Gibraltar, a British overseas territory on Spain's southern tip.
Authorities in Gibraltar last Thursday said they suspected the 330-metre Grace 1, which is capable of carrying two million barrels of oil, was carrying crude to Syria in violation of EU sanctions.
Tehran denies this and claims the vessel was intercepted in international waters.
Officials in Gibraltar said the cargo was believed to be destined for Syria.
Damascus is subject to EU sanctions while the US has its own sets of trade restrictions on Iranian oil.