U.K. says Iran attempted to block its oil tanker

Three Iranian vessels tried to block the passage of a BP-operated tanker through the Strait of Hormuz but withdrew after warnings from a U.K. navywarship, the U.K. government said on Thursday.

The standoff 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 behaviour.

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 spokesperson said in a statement.

In a statement issued Thursday, Vice-Admiral Jim Malloy said the U.S. Navy's Fifth Fleet is aware of Iran's"unlawful harassment and attempts tointerfere with the passage of the U.K.-flagged merchant vessel British Heritage" on Wednesday.

Malloy said the Fifth Fleet isworking closely with the British Royal Navy as well as regional and global partners to preserve and defend the freedom of navigation.

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.

On Thursday,Gibraltar police said they arrested the master and chief officer of the Grace 1 for breaching EU sanctions following a protracted search of the vessel where documents and electronic devices were seized.

The Grace 1 is still detained as the investigation continues, police said.

A spokesperson for U.K.Prime Minister Theresa May expressed concern over the incident in the strait.

"We have a long-standing maritime presence in the Gulf. We are continuously monitoring the security situation there and are committed to maintaining freedom of navigation in accordance with international law," the spokesperson said.

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.

Incident in 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 Strait of Hormuz theworld's most important oil artery links Middle East oil producers with markets in Asia, Europe, North America and beyond. It is 33 kilometres wide at its narrowest point but the shipping lane is just three kilometres wide in either direction.

Shipping tracker data showed the U.K.-flagged crude oil tanker Pacific Voyager operated by Mitsui OSK Lines took a similar route to the British Heritage on Wednesday through the Strait of Hormuz. Refinitiv data shows four other U.K.-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, its 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.

The longtime foes say publicly they want to avoid war, but the risk of direct confrontation has been rising. Last month, Iran shot down a U.S. drone near the Strait of Hormuz.

Trump aborted a retaliatory military strike, saying it could have killed 150 people, and signaled he was open to talks with Tehran without preconditions.

The United States hopes to enlist allies over the next two weeks or so in a military coalition to safeguard strategic waters off Iran and Yemen, Marine General Joseph Dunford, the chairman of the Joint Chiefs of Staff, said on Tuesday.

European parties to the nuclear accord Britain, France and Germany have sought to keep the deal alive but Iran has said it will take further steps away from the accord unless it is allow to resume normal oil sales.