A sailboat sails
A sailboat sails as a boat approaches to the boarding ladder of the Iranian oil tanker Grace 1 as it stays anchored after it was seized last month by British Royal Marines in the Strait of Gibraltar. Image Credit: Reuters

The United States moved on Thursday to halt the release of an Iranian supertanker detained in Gibraltar for breaching international sanctions on oil shipments, thwarting efforts by authorities in London and the British overseas territory to defuse tensions with Tehran.

The Gibraltar government confirmed earlier media reports that the U.S. Department of Justice had sought to extend the detention of the oil tanker Grace 1, prompting the Supreme Court in the territory to adjourn a scheduled decision on whether to release the ship until later in the day.

"The U.S. Department of Justice has applied to seize the Grace 1 on a number of allegations, which are now being considered," the Gibraltar government said in a statement, adding that the matter would be reviewed by the court at 4 p.m. local time.

The Justice Department did not immediately respond to a request for comment.

Britain's Foreign Office said in a statement that the "investigations conducted around the Grace 1 are a matter for the government of Gibraltar" and that it could not comment further as the investigation was ongoing.

2.1m

barrels of Iranian crude oil contained in Grace 1

Prime Minister Boris Johnson's Downing Street office said that Iran was discussed during the U.K. leader's meeting with Donald Trump's national security adviser John Bolton earlier in the week, though no details were released on the talks.

While there was no immediate reaction from Tehran, the U.S. move likely will further stir tensions in the Arabian Gulf.

The detention of the Grace 1 saw Iran seize the British-flagged oil tanker Stena Impero, which remains held by the Islamic Republic. Analysts had hoped the release of the Grace 1 by Gibraltar would see the Stena Impero similarly released.

In past weeks, the Arabian Gulf region has seen six attacks on oil tankers that the U.S. has blamed on Iran and the downing of a U.S. surveillance drone by Iranian forces. Iran has denied being behind the tanker attacks.

Iran also has seized other oil tankers.

The Grace 1, carrying 2.1 million barrels of Iranian crude oil, was seized last month in a British Royal Navy operation off Gibraltar. The vessel was suspected of violating European Union sanctions on oil shipments to Syria, and its seizure deepened international tensions in the Persian Gulf. Iran called the seizure by Gibraltar an "act of piracy."

Signaling preparations for the expected release of the ship, the captain, an Indian national, and three officers of the Grace 1 had been released from detention, a Gibraltar government spokesman told The Associated Press. The spokesman was not authorized to be identified by name in the media.

As speculation mounted over the Grace 1's release, a lawyer representing the territory's General Attorney Michael Llamas announced during a Thursday morning hearing at the Gibraltar Supreme Court that the U.S. had moved at the eleventh hour.

Speaking in court, Chief Justice Anthony Dudley said that were it not for the U.S. move, "the ship would have sailed," the Gibraltar Chronicle reported.

This is the second time the Trump administration has moved to seize a ship in recent months. In May, the Justice Department announced that it had seized a North Korean cargo ship used to supply coal to the isolated nation in violation of international sanctions.

Tanker seizures: what we know

The seizure is one of several in recent weeks that have ratcheted up tensions between Iran and its foe the United States and its allies.

Here’s what we know about the vessels involved:

Grace 1

Gibraltar police and customs officers aided by British Royal Marines intercepted the Grace 1 supertanker on July 4, as it passed through the strait between the territory on Spain’s southern tip and North Africa.

Officials in Gibraltar and the United States suspect the 330-metre (1,000 foot) Panama-flagged ship was destined for Syria, in breach of separate sets of EU and US sanctions.

The Indian captain of the vessel, which was carrying 2.1 million barrels of oil, later told the BBC a military helicopter landed on its deck before the Royal Marines boarded.

Iran has called the seizure of the tanker “maritime piracy” and warned at the time that it would not let its detention go unanswered.

The Islamic republic has not officially disclosed the tanker’s destination but it has repeatedly denied it was headed for Syria.

MT Riah

Iran’s Islamic Revolutionary Guard Corps said its forces detained a “foreign tanker” in Gulf waters on July 14 for allegedly smuggling contraband fuel.

The tanker was seized south of the Iranian island of Larak in the strategic Strait of Hormuz, the Guards’ official website said.

“With a capacity of two million litres and 12 foreign crew on board, the vessel was en route to deliver contraband fuel received from Iranian boats to foreign ships,” it said.

TankerTrackers reported at the time that the Panamanian-flagged MT Riah, used in the strait for fuelling other vessels, had crossed into Iranian waters, and at that point its automatic identification system stopped sending signals.

Stena Impero

Revolutionary Guards surrounded the British-flagged Stena Impero with attack boats before rappelling onto the deck of the oil tanker in the Strait of Hormuz on July 19.

The 183-metre ship is impounded at the Iranian port of Bandar Abbas for breaking “international maritime rules” - by allegedly failing to respond to distress calls and turning off its transponder after hitting a fishing boat.

Eighteen of the Swedish-owned tanker’s 23 crew are Indian, and the rest are from the Philippines, Latvia, and Russia.

The seizure came hours after a Gibraltar court said it would extend the detention of the Grace 1.

Britain quickly called on Iran to release the Stena Impero, saying it was seized illegally in an “utterly unacceptable” gambit.

But Tehran said the seizure was a legal measure and further investigations were required, denying it was a tit-for-tat move as London suggested.

Iran and Britain have both so far ruled out the possibility of an oil tanker swap deal.

Unknown vessel

Iran seized another ship on July 31 with seven foreign crew onboard, claiming it was smuggling around 700,000 litres of fuel.

The Guards said the ship was transferred to Bushehr province and handed over to authorities, claiming the vessel was en route to deliver fuel to Gulf Arab states.

The vessel’s identity and the nationality of its crew were not revealed at the time of its seizure.