GENEVA/LONDON — 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.

Iran's Revolutionary Guards posted a video online showing speedboats pulling up alongside the Stena Impero tanker, its name clearly visible.

Troops wearing ski masks and carrying machine guns rappelled to its deck from a helicopter, the same tactics used by British Royal Marines to seize an Iranian tanker off the coast of Gibraltar two weeks ago.

Friday's action in the global oil trade's most important waterway has been viewed in the West as a major escalation after three months of confrontation that has already taken Iran and the United States to the brink of war.

It follows weeks of threats from Tehran to retaliate for Britain's seizure of the Iranian tanker Grace 1, accused of violating sanctions on Syria.

London wants to de-escalate tensions with Tehran, Foreign Secretary Jeremy Hunt said Saturday following a meeting of the UK's emergencies committee over Iran's seizure of a British-flagged ship in the Gulf.

The meeting "reaffirmed UK desire to de-escalate," Hunt said in a tweet, adding that the ship Stena Impero was seized in Omani waters "in clear contravention of international law."

British Defense Secretary Penny Mordaunt called the incident a "hostile act".

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.

A spokesman for Iran's Revolutionary Guards, Brig.-Gen. 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. No British warship was visible in the video posted by the Guards.

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.

Zarif told Britain's Hunt that the ship must go through a legal process before it could be released, Iran's ISNA news agency reported.

The strait, between Iran and the Arabian peninsula, is the sole outlet for exports of most 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 fulfill promises to Iran of providing alternative means for it to access world trade.

Earlier Hunt 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 US drone and Trump ordered retaliatory air strikes, only to call them off just minutes before impact.

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". 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. — Reuters