– Britain mobilized a diplomatic broadside — but no immediate military action — against Iran on Saturday in retaliation for its seizure of the Stena Impero, a British-flagged tanker, in the Strait of Hormuz. Iran's top diplomat in London was summoned to the foreign office, while British Foreign Minister Jeremy Hunt expressed his "extreme disappointment" in a phone call to Mohammad Javad Zarif, his Iranian counterpart.

British ships were also warned to temporarily avoid the strategic Middle East waterway, and top government officials were looking for ways to stem the rising tensions while branding Iran's seizure of the British-flagged vessel an "illegal act."

"We are looking for ways to de-escalate the situation," Hunt said. "But we are also very clear that we will do what it takes to ensure the safety and security of British and international shipping."

For the second time in 24 hours, the British government convened an emergency session to discuss the seizure of the tanker in Omani waters "in clear contravention of international law," Hunt said. "This is totally and utterly ­unacceptable."

He said Parliament would be updated Monday about any further measures that will be taken, but he said the threat level has already been raised to the highest rank.

Only last Saturday, Hunt said Iranian officials had reassured him that they wanted to de-escalate tensions following the British detention of the Grace 1, an Iranian-affiliated ship, off the coast of Gibraltar.

The vessel was believed to have been carrying crude oil destined for Syria, in violation of E.U. sanctions.

"This has to be about actions not words if we are to find a way through," Hunt wrote on Twitter. "British shipping must & will be protected."

Hunt said that, following his conversation with Zarif, and from reading other reports, it was now clear to him that Iran saw Friday's seizure of the Stena Impero as a "tit for tat situation."

"Nothing could be further from the truth," Hunt added.

He also stressed that Britain was not seeking to emulate the "maximum pressure strategy" adopted by the Trump administration. "They have a different approach to dealing with Iran than us and France and Germany," Hunt said. "We continue to support the Iran nuclear deal. This is about the safety of British and international shipping in one of the most important seaways in the world."

Opposition Labor Leader Jeremy Corbyn also sought to distance himself from Trump's stance toward Iran. "The U.K. tanker under Iranian control, and its crew, must be released," Corbyn wrote on Twitter. "Escalation risks a deeper conflict, all sides must show restraint. Trump tearing up the Iran nuclear deal has fueled confrontation. Its negotiated reinstatement is essential to defuse threat of war in the gulf."

Meanwhile, Iran released a video of the seizure operation on Saturday.

The clip, published by Iranian news agencies, depicted a helicopter hovering over the Stena Impero (its name can be seen on the port side bow) before showing masked Iranian commandos rappelling onto the ship's deck to the cheers of an Iranian marine officer filming the operation from a nearby speedboat.

Later, other Iranian patrol boats can be seen flanking the ship. Iranian officials from various branches of government gave different justifications for holding the vessel.

Allah-Morad Afifipoor, head of Iran's Ports and Maritime Organization in Hormozgan Province, the southern Iranian province where the ship is docked in the port of Bandar Abbas, said the tanker had collided with a fishing boat.

The British tanker, which Afifipoor said had no cargo, did not respond to repeated requests for contact from the fishing boat, whose crew then contacted maritime ­authorities. The vessel was intercepted and taken to the port for an investigation, which began Saturday, he said.

"All its 23 crew members will remain on the ship until the investigation is over," Afifipoor said in an interview with Iranian news channels. "If necessary, and at the request of judicial authorities, the crew may be summoned for technical and specialist interviews."

A spokesman for Iran's Islamic Revolutionary Guard, Sardar Sharif, said that the ship was moving in a lane designated for exiting the Strait rather than the entrance corridor. The ship had also polluted water around it, said Sharif, and had turned off its Automatic Identification System, which is broadcast via onboard transponders and is used to avoid collisions.

Foreign minister Zarif, meanwhile, tweeted that "unlike the piracy in the Strait of Gibraltar, [Iran's] action in the Persian Gulf is to uphold maritime rules," in a reference to the seizure by British authorities of the Grace 1 tanker off Gibraltar earlier this month.