DUBAI — Iran’s capture of a British oil tanker risked spiraling into a major international crisis on Saturday, with neither side backing down from a standoff that exposes the world’s vulnerability to Tehran’s determination to resist U.S. sanctions.

Britain warned there would be “serious consequences” should Iran fail to release the tanker, seized Friday in the Strait of Hormuz — the narrow and crowded waterway through which a fifth of the world’s oil passes.

Iran, however, did not appear rattled. It said Iranian forces had fulfilled the threats to retaliate for Britain’s seizure of an Iranian oil tanker in the Mediterranean earlier this month for suspected violations of European Union sanctions against Syria. 

The European sanctions are unrelated to the new U.S. sanctions against Iran that were imposed after President Trump withdrew last year from the Iran nuclear deal — against the wishes of other signatories, including Britain — in an effort to force Tehran to renegotiate the terms of the deal.

President Trump on July 19 confirmed that Sen. Rand Paul (R-Ky.) is helping with diplomacy with Iran. (The Washington Post)

But the seizure of the British tanker sent a message to the world that Iran has the ability to interdict commercial shipping in the Strait of Hormuz whenever it chooses, despite U.S. and British naval reinforcements dispatched to the area to protect international shipping. 

“The Iranians are putting the world on notice that the U.S. policy of pulling out of the nuclear agreement and imposing sanctions on Iran is risking a regional war,” said Riad Khawaji of the Dubai-based Inegma security consultancy.

Iran could also sense that the British government — in disarray over Brexit and a change of leadership within days — was in “no shape to go to war,” he said. 

A video of the tanker capture, released by Iranian news outlets, showed uniformed men wearing ski masks aboard a helicopter hovering over the deck of the tanker. Iranian fastboats circled below. The tanker’s name, Stena Impero, is clearly visible on the vessel’s hull. 

Then, in video from one of the boats, five men are seen descending onto the deck of the vessel. A voice off-camera shouts “God is Great!”

Iran’s semiofficial Fars News Agency claimed that a British warship in the area attempted to prevent the interception of the Stena Impero. Britain’s Defense Ministry said the British warships dispatched to the area earlier this month to protect British shipping were at least an hour away at the time. There are no British or other foreign warships visible in the video.

British Defense Secretary Penny Mordaunt described the detention as a “hostile act” and the Foreign Office summoned the Iranian charge d’affaires in London to express Britain’s dismay.

The seizure constitutes the most serious escalation yet since Iran shot down an American drone in the area last month, prompting President Trump to consider, then call off, a direct strike on Iran. It suggests Iran is prepared to go to new lengths in its quest to resist the impact of the new U.S. sanctions, which U.S. officials say are designed to reduce Iran’s oil exports to “zero.”

Britain stressed that it is not considering military action to secure the return of the tanker. Foreign Secretary Jeremy Hunt said in a post on Twitter that any response would be “considered but robust.” Later, he tweeted that Britain sought to “de-escalate” the situation. 

“We are not looking at military options. We are looking at a diplomatic way to resolve this situation,” Hunt told a Sky News reporter. However, he added, “there will be serious consequences if we are not able to resolve it quickly.”

The British government urged all British shipping vessels to stay away from the Strait of Hormuz, the route for all ships transporting oil and other products from ports in the Persian Gulf. 

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

The Stena Impero is now being held at the Iranian port of Bandar Abbas, along with its 23-member crew, Iranian media said. The majority of the crew members are Indian nationals, but there are also Latvian, Fiji and Filipino nationals among them, according to the ship’s operator. There were no British citizens aboard the tanker, Hunt said.

A port official quoted by Fars claimed that the tanker was detained because it had a collision with a fishing boat. Another Iranian news agency, Tasnim, which is linked to the Revolutionary Guard Corps, made no mention of an accident.

Rather, it said, the ship was detained for violations of maritime laws, including sailing on the wrong side of the waterway and polluting the seas by dumping crude oil.

But a senior Iranian official made it clear that the main motive for the vessel’s detention was to retaliate for the British navy’s seizure of an Iranian tanker, the Grace 1, which was detained off the coast of Gibraltar at the mouth of the Mediterranean earlier this month. Hours before Iran’s seizure of the British tanker, a Gibraltar court extended by 30 days the time the authorities may continue to detain the Iranian tanker.

“The rule of retaliation is something that’s recognized within international law and is used in relation to wrong measures taken by a government,” said Abbas Ali Kadkhodaei, the spokesman for Iran’s powerful Guardian Council, according to Iran’s official IRNA news agency.

Unlike Tehran and Washington, Britain maintains diplomatic relations with Iran, and their conversations concerning the detained tankers played out on Twitter. Hunt said on his Twitter account that he spoke to Iran’s foreign minister, Mohammad Javad Zarif, to express his “disappointment” that Iran appeared to have reneged on a promise to de-escalate the tensions surrounding Britain’s detention of the Grace 1 tanker in Gibraltar.

Zarif called on Britain to “cease being an accessory” of what he called the economic terrorism of the United States.

Hunt also spoke on the phone with U.S. Secretary of State Mike Pompeo on Friday night, and British and U.S. officials were in contact, according to officials from both countries.

In Guayaquil, Ecuador, Pompeo told reporters that the United States remained open to talks with Iran “across a broad spectrum of issues with no preconditions.”

“We’ve done everything we can to create the space for this,” he added, “but to date we have seen no indication that the Iranians are prepared to fundamentally change the direction of their nation.”

Booth reported from London. Karla Adams in London, Karen DeYoung in Guayaquil, Ecuador, and Brian Murphy and Carol Morello in Washington contributed to this report.