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.
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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."