The stand-off followed a warning by U.S. President Donald Trump that U.S. sanctions on Iran would soon be "substantially" increased as part of Washington's drive to curb Iran's nuclear activities and regional behavior.

Britain urged Iran to "de-escalate the situation in the region" after the incident involving British Heritage, which is operated by BP under an Isle of Man flag.

"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," a British government spokesman said in a statement.

The incident came almost a week after British Royal Marines boarded an Iranian tanker, the Grace 1, off Gibraltar and seized it on suspicion that it was breaking EU sanctions by taking oil to Syria.

Iran's armed forces chief of staff, Major General Mohammad Bagheri, had said the British seizure would not go unanswered but the Islamic Republic denied it had sought to stop the British Heritage.

Iranian Foreign Minister Mohammad Javad Zarif dismissed the British report as "worthless", according to the semi-official Fars news agency.

Key Shipping Lane

A BP spokesman said the oil major's top priority was the safety and security of its crews and vessels, adding: "While we are not commenting on these events, we thank the Royal Navy for their support."

BP CEO Bob Dudley, asked about the tensions in the Gulf at an event at London's Chatham House on Wednesday evening, said: "We've got to be super careful about our ships".

The world's most important oil artery links Middle East oil producers with markets in Asia, Europe, North America and beyond. It is just 21 miles (33 km) wide at its narrowest point but the shipping lane is just two miles (three km) wide in either direction.

Shipping tracker data showed the UK-flagged crude oil tanker Pacific Voyager operated by Mitsui OSK Lines Ltd took a similar route to the British Heritage on Wednesday through the Strait of Hormuz. Refinitiv data shows four other UK registered tankers are currently present in the Gulf.

Tensions in the Gulf have been rising over recent weeks as Iran began to move away from the terms of the 2015 nuclear accord it struck with world powers.

The United States withdrew from the pact last year and extended sanctions against Iran, effectively driving Iran from mainstream oil markets and forcing it to find unconventional ways to sell crude, it's main revenue earner.

That has deprived Tehran of the economic benefits Iran was to accrue in return for curbing its nuclear program, and the Islamic Republic says it will only return to full compliance once sanctions are lifted and Washington rejoins the pact.

REUTERS