Foreign minister Jeremy Hunt said on Sunday Britain is "almost certain" Iran was behind attacks on oil tankers in the Gulf of Oman, adding that London did not believe anyone else could have done it.
On Saturday, Iran summoned the British ambassador to Tehran after London blamed it for the attacks, the semi-official Students News Agency ISNA reported.
Asked whether Iran was behind the attacks on the tankers, Hunt told the BBC's Andrew Marr show: "We have done our own intelligence assessment and the phrase we used is almost certain ... We don't believe anyone else could have done this."
"We are urging all sides to de-escalate."
The U.S. military said black-and-white footage it filmed from a U.S. aircraft showed Iran's Guards on a patrol boat drawing up to the Kokuka Courageous and removing an unexploded limpet mine from its hull.
The Japanese-owned tanker, abandoned by its crew, was being towed to a port in the United Arab Emirates on Friday, after a Dutch firm said it had been appointed to salvage the ships.
The second tanker, the Front Altair, which was set ablaze by a blast, was still languishing at sea, although the fire that had charred the hull had been put out.
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Iranian military fast-boats in the Gulf of Oman were preventing two privately owned tug boats from towing away the Front Altair, a U.S. official said on Friday.
China, the European Union and others have called for restraint from all sides. In a notable signal that close U.S. allies are wary of Washington's position, Germany said the U.S. video was not enough to apportion blame for Thursday's attack.
Over the weekend, U.S. officials claimed that Iran had targeted and missed a U.S. drone flying over the Gulf of Oman shortly after the Thursday attack on the two tankers.
The drone was reportedly responding to a distress signal from one of the tankers when Iran fired a surface-to-air missile at the drone, the officials said.