The British navy said it prevented three Iranian paramilitary vessels from impeding the passage of a British oil tanker in the Strait of Hormuz early Thursday, a day after Iran's president warned of repercussions for the seizure of its own supertanker.
Iran's Revolutionary Guard denied the allegations, saying if it had received orders to seize any ships it would have done so immediately.
A US aircraft was in the area at the time and the military has video imagery, a US official said, speaking on condition of anonymity to discuss security matters.
The incident came at a time of heightened tensions over Iran's unraveling nuclear agreement with world powers, signed in 2015. Iran recently began breaching uranium enrichment limits in response to the Trump administration's withdrawal from the agreement last year and its imposition of sweeping sanctions.
In recent months, the US has sent thousands of additional troops, an aircraft carrier, B-52 bombers and advanced fighter jets to the region.
Washington has blamed Iran for a series of mysterious attacks on oil tankers - charges that have been denied by Tehran - and said it shot down an American military surveillance drone in international airspace. Iran said it downed the drone after it veered into its own airspace.
The UK said in a statement that the British naval vessel HMS Montrose had been accompanying the commercial ship, British Heritage, through the narrow Strait of Hormuz, a crucial waterway for energy shipments.
"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 statement said.
"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region," it added.
The HMS Montrose is on a three-year mission at the British navy's support facility in Bahrain, the hub of its naval operations east of the Suez Canal.
Russia and China, both signatories to the nuclear agreement along with Britain, France and Germany, called for restraint.
Kremlin spokesman Dmitry Peskov said "freedom of navigation should be ensured in the Persian Gulf and the Strait of Hormuz."
The US 5th Fleet in Bahrain declined to comment on the incident.
US Central Command spokesman Capt. Bill Urban said CENTCOM was aware of reports of "harassment and attempts to interfere with" the passage of the British Heritage near the Strait of Hormuz by the Islamic Revolutionary Guard's naval forces.
Threats to freedom of navigation require an international solution, Urban said.
Maritime security risk firm Dryad Global described the British Heritage as an oil tanker operated by BP and registered in the Isle of Man. Lloyd's List, a publication specializing in maritime affairs, said Shell had chartered the ship from BP.
Lloyd's List said the British Heritage had diverted from its route to load its 140,000-ton cargo of crude at Basra, Iraq, as planned on July 4, the same day an Iranian supertanker was intercepted off Gibraltar, a British overseas territory. It said the vessel instead headed to Saudi waters where it had remained for several days.
Since July 2, at least 20 British-flagged ships have sailed through the Strait of Hormuz, according to Lloyd's List Intelligence data.
About 20 per cent of all oil traded worldwide passes through the Strait of Hormuz from Middle East producers. Iran has periodically threatened to close the shipping lane if it is prevented from exporting its own oil. The US sanctions have largely shut down its oil exports.
Despite the latest incident, the overall threat level for non-British ships using the critical waterway has not changed significantly, said Jakob P. Larsen, head of maritime security for BIMCO, the largest international association representing ship owners.
US Secretary of State Mike Pompeo has asked Mideast allies like Saudi Arabia and the United Arab Emirates in past weeks to contribute financially and militarily to a Trump administration proposal called the Sentinel Program a coalition of nations working with the US to preserve maritime security in the Persian Gulf and keep eyes on Iran.
The semi-official Fars news agency carried a statement from the Iranian Revolutionary Guard's navy on Thursday denying any incident had occurred. "In the last 24 hours, there were no clashes with alien floats, especially British boats," the statement said.
Iran's Foreign Minister Mohammad Javad Zarif dismissed the British allegations as "worthless," saying the claims "are being made to create tension," Fars reported.
President Hassan Rouhani had recently warned that Britain would face "repercussions" over the seizure of the Iranian supertanker last week.
Authorities in Gibraltar suspect the vessel was breaching European sanctions on oil shipments to Syria.
Iran had also summoned the British ambassador over what it called the "illegal interception" of the ship.
The operation to seize the Iranian tanker took place one week ago at the request of the US Gibraltar port and law enforcement agencies were assisted by Britain's Royal Marines in the seizure.
The tensions have increased since President Donald Trump's decision last year to withdraw from the landmark nuclear agreement negotiated by the Obama administration.
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