The British navy said it prevented three Iranian paramilitary vessels from impeding the passage of a British commercial ship 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.
The incident came at a time of heightened tensions over Iran's unraveling nuclear agreement with world powers. 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 U.S. has dispatched thousands of additional troops, an aircraft carrier, bombers and advanced fighter jets to the region. Washington has blamed Iran for a series of mysterious attacks on oil tankers — charges denied by Tehran — and said it shot down an American drone in international airspace. Iran said it downed the drone after it veered into its own airspace.
The U.K. government said in a statement that British navy vessel HMS Montrose had been accompanying the commercial ship, British Heritage, through the narrow Strait of Hormuz. 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.
"We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region," it added.
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The U.S. 5th Fleet and Central Command declined to comment on the incident.
Around 20% of all oil traded worldwide passes through the Strait of Hormuz. Iran has periodically threatened to close the shipping lane if it is prevented from exporting its own oil. The U.S. has vowed to keep the strait open to commercial traffic.
The semi-official Fars news agency carried a statement from the Iranian Revolutionary Guard's navy early Thursday saying there were no clashes with foreign ships, "especially British boats."
Iran's Foreign Minister Mohammad Javad Zarif dismissed the British allegations as "worthless," saying the claims "are being made to create tension," the agency reported.
Iran's president, Hassan Rouhani, had recently warned that Britain would face "repercussions" over the seizure of an Iranian supertanker last week that authorities in Gibraltar, a British overseas territory, suspect 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 exactly one week ago at the request of the United States. Gibraltar port and law enforcement agencies were assisted by Britain's Royal Marines in the ship's seizure.
The tensions are rooted in President Donald Trump's decision last year to withdraw from the landmark nuclear agreement negotiated by the Obama administration. Trump has since re-imposed crippling sanctions on Iran, including on its vital oil exports, exacerbating an economic crisis that has sent the currency plummeting.
The remaining parties to the nuclear deal — Russia, China, Germany, France, Britain and the European Union — have been unable to meet Tehran's demands for economic assistance to offset the American sanctions.
In recent weeks, Iran began breaching the limits of the deal, both on the permitted stockpile of low-enriched uranium and the permitted level of uranium enrichment.
It also set an early September deadline for world powers to save the agreement, saying it would otherwise take a third step in going beyond the deal's limits.
Iran maintains it is justified in breaching the limitations because the U.S. already broke the deal with its unilateral withdrawal last year.
Associated Press writers Robert Burns and Andrew Drake contributed to this report.