London was reportedly getting set to impose new sanctions on Iran Saturday, as European powers urged Tehran to release a British-flagged tanker it seized in the Strait of Hormuz.

The Islamic Revolutionary Guard Corps said it seized the Stena Impero Friday for breaking “international maritime rules” in the strait, a choke-point for around a third of the world’s sea-borne oil.

British Foreign Secretary Jeremy Hunt warned of “serious consequences” for the move and on Sunday was expected to announce a series of diplomatic and economic measures aimed at Iran, the Telegraph reported.

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The measures could include moves to freeze assets and London could also push for the EU and UN to reimpose sanctions lifted under the 2015 nuclear deal, according to the report.

Hunt said Britain’s response “will be considered but robust.”

In comments on Twitter on Saturday, he said he spoke with Iran’s foreign minister and expressed extreme disappointment that the Iranian diplomat had assured him Iran wanted to de-escalate the situation but “they have behaved in the opposite way.”

Speaking to reporters later Saturday after an emergency government meeting, Hunt said the “totally and utterly unacceptable” interception of the British-flagged Stena Impero “raises very serious questions about the security of British shipping and indeed international shipping” in the Strait of Hormuz.

The tanker was impounded off Bandar Abbas port for allegedly failing to respond to distress calls and turning off its transponder after colliding with a fishing vessel, authorities said.

Britain said Iran had seized two ships in the Gulf, but the British owner of the Liberian-flagged Mesdar said it had been released after being temporarily boarded by armed personnel.

That came hours after a court in Gibraltar said it would extend by 30 days the detention of an Iranian tanker seized by British authorities two weeks ago on allegations of breaching EU sanctions against Syria.

Hunt said Friday’s incident showed “worrying signs Iran may be choosing a dangerous path of illegal and destabilizing behavior.”

His government advised British ships to avoid the Strait of Hormuz for “an interim period.”

“We remain deeply concerned about Iran’s unacceptable actions which represent a clear challenge to international freedom of navigation,” a spokeswoman said after an overnight meeting of Britain’s COBRA emergencies committee.

Germany and France urged Iran to release the tanker, whose seizure Berlin called a “dangerous further aggravation of an already tense situation.”

The Guards also said Thursday they had seized another “foreign tanker” and its crew days earlier for allegedly smuggling fuel, without giving further details.

Tit-for-tat

Hunt said Iranian officials “see this as a tit-for-tat situation, following Grace1 being detained in Gibraltar. Nothing could be further from the truth.”

The view from Iran was different.

In comments on Twitter on Saturday, Iran’s Foreign Minister Mohammad Javad Zarif characterized the seizure of Iran’s tanker July 4 as “piracy.” Politician and former Guard commander, Maj. Gen. Mohsen Rezai, wrote that Iran was not seeking conflict, “but we are not going to come up short in reciprocating.”

The spokesman for Iran’s Guardian Council, Abbas Ali Kadkhodaei, was also quoted in the semi-official Fars news agency describing Friday’s seizure as a legal “reciprocal action.” The council rarely comments on state matters, but when it does it is seen as a reflection of the supreme leader Ayatollah Ali Khamenei’s views. The council works closely with Khamenei, who has final say on all state matters.

Tensions in the Gulf have soared since May, with US President Donald Trump calling off air strikes against Iran at the last minute in June after the Islamic Republic downed a US drone.

Washington has also blamed Iran for multiple attacks on tankers in the Gulf.

Trump said Friday’s incident “only goes to show what I’m saying about Iran: trouble. Nothing but trouble.”

The escalation comes more than a year after Washington unilaterally withdrew from the 2015 Iran nuclear agreement and began ratcheting up sanctions against Tehran.

Earlier this month, Iran exceeded the deal’s caps on uranium enrichment, aiming to pressure the remaining parties to make good on promises to help prop up its economy.

Tehran has repeatedly threatened to close the Strait of Hormuz if attacked.

Hunt warned that “if this situation is not resolved quickly there will be serious consequences.”

But he told Sky News that “we’re not looking at military options, we are looking at a diplomatic way to resolve the situation.”

‘Seized for further investigation’

The Stena Impero had been heading for Saudi Arabia on Friday when it hit a fishing vessel, according to port authorities at Bandar Abbas.

Allah-Morad Afifipoor, director-general of the Hormozgan province port and maritime authority, said experts would investigate the incident.

The Swedish-owned tanker “has 23 crew and they are all on the ship,” he said, quoted by Fars news agency.

The Philippines said 18 Indians, three Russians, a Latvian and a Filipino were aboard.

Both Manila and New Delhi said they had contacted Tehran to seek their nationals’ release.

Afifipoor said the fishermen had issued a distress call after the collision, contacting the port authority when they “didn’t receive any response.”

“One of the reasons the British tanker was seized for further investigation was that for a period of time it was moving on its route with its transponder turned off,” he told ILNA news agency.

Tracking service MarineTraffic showed the Stena Impero had last signaled its location near the island of Larak at 9:00 pm.

Its owner said the ship was transiting the Strait and was “international waters” when it was “attacked by unidentified small crafts and a helicopter,” the owner said.

In a dramatic video released by the Revolutionary Guard, several small Guard boats can be seen surrounding the larger tanker as it moves through the strait. Above, a military helicopter hovers and then several men wearing black masks begin to rappel onto the ship.

The high-quality video was shot with at least two cameras, one from a speed boat-like vessel and one from the chopper, which captured the fatigue-clad men as they prepared to slide down a rope and also took aerial footage of the tanker.

Drone dispute

The incident came as Trump and American officials insisted, despite denials from Tehran, that the US military had downed an Iranian drone that was threatening an American naval vessel in the Strait.

Trump said the drone had been threatening amphibious assault ship USS Boxer.

The Revolutionary Guards released video footage they said disproved the US claims.

The seven-minute video, apparently shot from high altitude, shows a convoy of ships the Guards said they were tracking as they passed through the Strait.

The vessels could not be immediately identified, although one resembles the USS Boxer.

As tensions soared, Iran’s archrival Saudi Arabia said it would once again host US troops to boost regional security.

The Pentagon said the deployment “ensures our ability to defend our forces and interests in the region from emergent, credible threats.”

The US military also said its patrol aircraft were monitoring the Strait, and announced a “multinational maritime effort” to ensure freedom of navigation in key waterways.