Iran's Revolutionary Guards posted a video online on Saturday showing speedboats pulling up alongside the detained British-flagged tanker Stena Impero and troops wearing ski masks and carrying machine guns rappelling to its deck from a helicopter.
Britain denounced the Iranian seizure of a British-flagged oil tanker in the Gulf as a "hostile act" on Saturday, rejecting Tehran's explanation that it had seized the vessel because it had been involved in an accident.
The name of the ship can be seen in the video.
Friday's action by Iranian Revolutionary Guards in the global oil trade's most important waterway has been viewed in the West as a major escalation in three months of confrontation that took Iran and the United States to the brink of war.
It came two weeks after Britain seized an Iranian tanker off Gibraltar accused of violating sanctions on Syria, an action that prompted numerous Iranian threats to retaliate.
British Defense Secretary Penny Mordaunt called the incident a "hostile act". Foreign Secretary Jeremy Hunt said he had expressed extreme disappointment by phone to his Iranian counterpart, Mohammad Javad Zarif. Britain also summoned the Iranian charge d'affaires in London.
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A spokesman for Iran's Revolutionary Guards, Brigadier-General Ramezan Sharif, said Tehran had seized the ship in the Strait of Hormuz despite the "resistance and interference" of a British warship which had been escorting it.
Iran's Fars news agency said the Guards had taken control of the Stena Impero on Friday after it collided with an Iranian fishing boat whose distress call it ignored.
The vessel, carrying no cargo, was taken to the Iranian port of Bandar Abbas. It would remain there with its 23 crew -- 18 of them Indians -- while the accident was investigated, Iranian news agencies quoted the head of Ports and Maritime Organization in southern Hormozgan province, Allahmorad Afifipour, as saying.
Iran's semi-official Tasnim news agency posted a video of the ship anchored at sea, its name clearly visible. Zarif told Britain's Hunt that the ship must go through a legal process before it could be released, Iran's INSA news agency reported.
The strait, between Iran and the Arabian peninsula, is the sole outlet for exports of the vast majority of Middle Eastern oil, and the seizure sent oil prices sharply higher. The United States, which tightened sanctions against Iran in May with the aim of halting its oil exports altogether, has been warning for months of an Iranian threat to shipping in the strait.
France, Germany and the European Union joined Britain in condemning the seizure.
The three big European countries are signatories to a 2015 nuclear deal between Tehran and world powers that Washington undermined by quitting last year, setting Iran's already fragile relations with the West on a downward spiral.
Under the pact, Iran agreed to restrict nuclear work in return for lifting sanctions. The European countries opposed the Trump administration's decision to abandon the agreement last year, but have so far failed to fulfil promises to Iran of providing alternative means for it to access world trade.
"Just spoke to ... Zarif and expressed extreme disappointment that having assured me last Saturday Iran wanted to de-escalate situation, they have behaved in the opposite way," tweeted British Foreign Secretary Hunt. "This has to be about actions not words if we are to find a way through."
Earlier he said London's reaction would be "considered but robust", and it would ensure the safety of its shipping.
On Friday, Hunt said the solution would be found via diplomacy and London was "not looking at military options." Britain's government said it had advised British shipping to stay out of the Hormuz area for an interim period.
The past three months of escalation have seen the United States and Iran come as close as ever to direct armed conflict. In June, Tehran shot down a U.S. drone and Trump ordered retaliatory air strikes, only to call them off just minutes before impact.
Since British Royal Marines abseiled from a helicopter off Gibraltar to seize the Grace 1 Iranian tanker on July 4, a number of Iranian officials had threatened to retaliate. After maintaining silence for more than a week, Iran's Supreme Leader, Ayatollah Ali Khamenei called the British action "piracy".
A senior politician and Revolutionary Guards commander, Major General Mohsen Rezai, said on Twitter that Iran was not looking for war, "but we are not going to come up short in reciprocating."
In New Delhi, India's foreign ministry said it was actively seeking the release of its nationals among the crew.
Operator Stena Bulk said on Friday the tanker had been "in full compliance with all navigation and international regulations", but could not be contacted. On Saturday it said it was preparing a request to visit the crew.
The vessel had been heading to a port in Saudi Arabia and suddenly changed course after passing through the strait.
The United States has blamed Iran for a series of attacks on shipping around the Strait of Hormuz. Tehran rejects the allegations. Washington also said it had this week downed an Iranian drone near where the Stena Impero was seized.
The United States is sending military personnel and resources to Saudi Arabia for the first time since the U.S. invasion of Iraq in 2003.