Britain says three Iranian vessels unsuccessfully tried to impede the passage of a British commercial vessel through the Strait of Hormuz.
The British government said in a statement Thursday that the Iranian vessels only turned away after receiving "verbal warnings" from a U.K. navy vessel accompanying the commercial vessel British Heritage.
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"HMS Montrose [frigate] 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 representative said in a statement.
"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region," the statement adds.
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Maritime security risk firm Dryad Global described the British Heritage as an oil tanker owned by BP and registered in the Isle of Man.
"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 government statement said.
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Iran's Foreign Minister Mohammad Javad Zarif dismissed as "worthless" the British allegation that Iran had sought to block the ship.
The incident followed President Donald Trump's warning he would soon "substantially" increase U.S. sanctions on Iran as part of a drive to curb Iran's nuclear programme and force Tehran to change its regional behaviour.
The U.S. 5th Fleet and Central Command declined to comment on the incident.
Iran's Revolutionary Guards rejected the claim that they tried to stop the British vessel, the semi-official Fars news agency reported.
According to the BBC, the British Heritage tanker was approached as it was sailing from the Persian Gulf into the Strait of Hormuz, near the island of Musa. The island and its territorial waters are disputed between Iran and the United Arab Emirates, but according to the the report, the British vessels never left international waters.
The incident marked the latest escalation of tensions in the Persian Gulf over the 2015 nuclear deal between Iran and world powers, which the Trump administration abandoned last year.
The incident occurred almost a week after British Royal Marines boarded an Iranian tanker, Grace 1, off Gibraltar and seized it on suspicion that it was breaking sanctions by taking oil to Syria.
Tensions between Iran and the United States and its allies have risen sharply since Washington stepped up economic sanctions against Iran and moved to bring the country's oil exports to zero as part of a "maximum pressure" policy to make Iran halt actions that it said undermined regional security.
Iran has responded to the sanctions by starting to breach limits put on its nuclear activities under a 2015 deal with world powers.
Several oil tankers were attacked in waters near Iran's southern coast in May and June, for which the United States blamed Iran. Tehran denied any involvement.
Last month, Iran shot down a U.S. drone near the Strait of Hormuz, prompting President Donald Trump to order retaliatory air strikes, only to call them off.
The U.S. officials, speaking on the condition of anonymity, said Wednesday's incident happened as British Heritage was at the northern entrance of the Strait of Hormuz.
"The Royal Navy HMS Montrose, which was also there, pointed it guns at the boats and warned them over radio, at which point they dispersed," one of the officials said.
"It was harassment and an attempt to interfere with the passage," the other official said.
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.
Key shipping lane
U.S. sanctions have effectively driven Iran from mainstream oil markets, depriving it of its main source of revenue and of the benefits it was meant to receive from its nuclear deal. Iran says it will return to full compliance with the agreement only if sanctions are lifted and Washington rejoins the pact.
BP CEO Bob Dudley, asked about the situation 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".
An escalation in the Strait of Hormuz, the main outlet for Middle East oil traded around the globe, could drive up crude prices.
Maritime security sources said Britain was aiming to protect shipping lanes but there was no formal policy of escorting all UK ships through the area. The Montrose was there to ensure the safe passage of UK flagged ships when needed, they added.
Ship tracking information from data firm Refinitiv shows four other UK registered tankers now in the Gulf.
Bob Sanguinetti, chief executive with the UK Chamber of Shipping trade association, told Reuters the situation was tense and called for a de-escalation.
"UK shipowners are in regular contact with the relevant authorities and agencies regarding the security situation in the region, and we are confident that the RN (Royal Navy) will provide the necessary support to their vessels," he said.
Oman, which hosts a joint British military base and shares the Strait of Hormuz with Iran, did not immediately comment. It has mediated between Tehran and the West and also allows the British and U.S. navies to use its ports on the Arabian Sea.
Britain, France and Germany have sought to avoid being dragged into U.S. sanctions but say Iran must return to full compliance with the 2015 nuclear deal they were instrumental in brokering.
They have so far avoided triggering a dispute resolution process contained in the deal. Iran says it could take new steps in the next two months, including restarting dismantled centrifuges and purifying uranium to a sharply higher threshold, unless it is allow to resume normal oil sales.
Francois Lecointre, the French armed forces chief, described the friction between the United States and Iran as a "clash of wills".
"I think it is under control now... I don't think it can spiral out of control but there can be escalation," he told CNews television.
DPA contributed to this report.