DUBAI — Iran posted a video on Saturday showing masked men descending from a helicopter onto the deck of the British tanker that was seized by Iranian forces in the Strait of Hormuz, as Britain warned of “serious consequences” should Iran fail to release the vessel.

In the video, posted by the semiofficial Iranian Fars news agency, uniformed men wearing black ski masks are seen gathered in the helicopter as fastboats encircle the Stena Impero tanker, its name clearly visible on the hull. In footage filmed from one of the boats, five men are then seen descending onto the deck of the vessel as a voice off camera shouts “God is Great.”

Iran’s seizure of the tanker on Friday evening constitutes the most serious escalation yet since Iran shot down an American drone near the waterway a month ago, prompting President Trump to consider, then call off, a direct strike on Iran. The publication of the video helped sustain the revived tensions in the Persian Gulf region, where Iran increasingly seems intent on provoking a showdown with the West.

The spokesman of Iran’s Guardian Council, Abbas Ali Kadkhodaei, was quoted as saying Friday’s interdiction of the Stena Impero tanker was “reciprocal action” in response to Britain’s detention earlier this month of an Iranian supertanker in the Mediterranean.

Kadkhodaei’s remarks, reported by Fars, also denounced the “illegitimate economic war” on Iran, an apparent reference to the harsh new sanctions imposed by the Trump administration that are designed to force Tehran to renegotiate the 2015 nuclear deal, reached under the Obama administration.

The Guardian Council, a powerful group that oversees internal matters such as elections, rarely comments on international affairs. But its declarations about the tanker seizure possibly reflect the views of Iran’s supreme leader, Ayatollah Ali Khamenei, and a willingness by Iran’s rulers to step up the brinkmanship.

Earlier, Iran said the Stena Impero tanker, with 23 crew members aboard, had been detained on the grounds that it failed to stop after colliding with a fishing vessel.

Britain, while saying it was not considering a military solution to the tanker’s seizure, noted a possible response would be “considered but robust.”

“We are not looking at military options. We are looking at a diplomatic way to resolve this situation,” said British Foreign Secretary Jeremy Hunt. However, he added, “there will be serious consequences if we are not able to resolve it quickly.”

Meanwhile, London is urging all British shipping to stay away from the strategic Strait of Hormuz at the mouth of Persian Gulf – the route for a fifth of the world’s oil.

In this instance, the ship’s seizure appears to be a fulfillment of Iran’s threats to retaliate for the detention of an Iranian supertanker earlier this month by the Royal Navy near Gibraltar. British authorities said the tanker was stopped on the grounds that it was violating European Union sanctions on Syria.

On Friday, the Gibraltar Supreme Court granted local authorities the right to continue to detain the Iranian tanker, the Grace 1, for another 30 days, as investigations continue into the destination of the oil it was carrying.

Hours later, the Stena Impero was surrounded by four speedboats and a helicopter before it was forced to change course and head toward Iranian waters, according to British officials and the ship’s operators. None of the ship’s 23 crew members are British, said Hunt. The ship’s operator said they are mostly Indian, Russian, Latvian and Filipino.

Iran’s foreign minister, Mohammad Javad Zarif, called the interception of the Iranian tanker off Gibraltar “piracy.”

“Our action in the Persian Gulf is to uphold int’l rules,” he wrote in a tweet on Saturday, calling on Britain to “cease being an accessory” to U.S. sanctions against Iran.

A second ship, the Mesdar, was also intercepted by speedboats at around the same time as the Stena Impero, but it was allowed to continue its journey, according to reports in Iran’s state-run media.

The Stena Impero is being held at the Iranian port of Bandar Abbas, according toFars. The news agency quoted port officials as saying the tanker refused to respond to appeals from the allegedly damaged fishing boat to stop and offer assistance, a violation of maritime laws.

However, the Tasnim news agency, which is close to the Islamic Revolutionary Guard Corps, made no mention of an accident. It reported that the ship was detained because it had switched off its GPS systems, was sailing on the wrong side of the waterway and was polluting the seas by dumping oil.

“Yesterday’s action in the Gulf shows worrying signs Iran may be choosing a dangerous path of illegal and destabilizing behavior after Gibraltar’s LEGAL detention of oil bound for Syria,” Hunt said on Twitter.

In Washington, President Trump said Friday the interception of the British ship proved his repeated assertions that Iran is “nothing but trouble.”

“It goes to show you I was right,” he said, adding: “It’s not American, it’s U.K. . . . Let’s see what happens.”

Hunt spoke on the phone with U.S. Secretary of State Mike Pompeo on Friday night, and officials in London and Washington were in touch throughout the night, according to U.S. and British officials. The British government asked Pompeo to hold back from public comments that might further inflame the situation, as London tries to resolve the crisis through diplomacy, according to a U.S. official who spoke on the condition of anonymity because of the sensitivity of the subject.

Friday’s interceptions signal a wider escalation by Iran in its two-month-old campaign of threats and attacks against United States and allied warships and commercial shipping in the vicinity of the Persian Gulf, as Tehran seeks to push back against the Trump administration’s imposition of tough new sanctions. Iran has denied U.S. allegations that it is responsible for most of the attacks.

The new tensions coincide with the arrival in the region of U.S. naval reinforcements aimed at securing the safety of shipping in the area. Among the U.S. warships that have arrived in the region is the USS Boxer, which on Thursday destroyed an Iranian drone that had approached dangerously close, according to Trump and the Pentagon.

Iran however rebutted that the drone had been brought down and on Friday broadcast footage of a warship that it claimed was the Boxer to demonstrate that the drone was still functional.

A different U.S. official who spoke on the condition of anonymity because of the sensitivity of the situation said the U.S. military has surveillance aircraft in the region watching what is happening. Naval Forces Central Command, which oversees U.S. military operations in the region, has been in contact with commercial U.S. ships in the region.

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