Officials in Gibraltar have allowed a detained Iranian supertanker to leave despite a last-minute US attempt to seize the vessel.

The Supreme Court in Gibraltar had delayed a decision to release the Grace 1 after the US Department of Justice made an application to extend the vessel’s detention, the Gibraltar government said earlier on Thursday.

But the Gibraltar Chronicle reports there was no US application before the court when the hearing resumed on Thursday afternoon and so the vessel was allowed to leave.

Grace 1
The Grace 1 was seized in an operation by the British Navy last month (Marcos Moreno/AP)

The Grace 1, carrying 2.1 million barrels of Iranian crude, was seized on July 4 in a British Royal Navy operation off Gibraltar. The vessel was suspected of violating European Union sanctions on oil shipments to Syria.

The detention of the Grace 1 saw Iran later seize the British-flagged oil tanker Stena Impero, which remains held by the Islamic Republic.

The Grace 1’s release comes after the US under President Donald Trump pulled out of Iran’s nuclear deal with world powers over a year ago, setting in motion a growing confrontation between Tehran and the West over its atomic programme.

Reacting to the latest developments, Iran’s foreign minister Mohammad Javad Zarif accused the US of trying to “steal our property on the high seas”.

He tweeted: “Having failed to accomplish its objectives through its #EconomicTerrorism — including depriving cancer patients of medicine — the US attempted to abuse the legal system to steal our property on the high seas,” and he described the Trump administration’s moves as a “piracy attempt”.

In recent weeks, the Persian Gulf region has seen six attacks on oil tankers that the US has blamed on Iran and the downing of an American surveillance drone by Iranian forces. Iran has denied being behind the tanker attacks, though it has seized other vessels.

Grace 1
Crew members on board the Grace 1 as it sits at anchor off Gibraltar (Marcos Moreno/AP)

Signalling preparations for the expected release of the Grace 1, its captain, an Indian national, and three officers were freed from detention early on Thursday.

Lina Khatib, head of the Middle East and North Africa programme at Chatham House, said the US request on the Grace 1 was based on its own imposed sanctions on Iran, and not the EU’s sanctions on oil exports to Syria.

“Although the US expects its European allies to abide by these sanctions, it is up to the Gibraltar authorities to assess the allegations presented by the US,” Ms Khatib said.

The US has been asking its allies to take part in a naval mission to protect shipping in the strategic Strait of Hormuz, at the mouth of the Persian Gulf, though European nations have been reluctant.

Britain has so far been the only one to express willingness to join a maritime security mission. It has also been giving UK-flagged vessels a naval escort since the Iranian Revolutionary Guard’s seizure of the Stena Impero.