IRAN has warned the US against any attempt to seize an oil tanker freed from Gibraltar after being detained last month.

The Iranian-controlled vessel, formerly known as Grace 1 but renamed by Iran as the Adrian Darya 1, is sailing through the Mediterranean after leaving the British territory late on Sunday despite an attempt by the US over the weekend to block its departure.

The ship is carrying 2.1m barrels of oil and is reported to be heading for a Greek port, which will take several days.

Iran has declared the decision to release the ship to be a victory over Washington.

Asked whether the US might renew its seizure request, Iranian foreign ministry has said “such an action... would endanger shipping safety” in open seas.

“We have issued a warning through official channels, especially the Swiss embassy, to American officials not to commit such an error because it would have heavy consequences,” Foreign Ministry spokesman Seyed Abbas Moussavi said on state television on Monday.

Switzerland represents US interests in Iran. Tehran has no official diplomatic relations with Washington.

However, Iran has indicated that the release of the tanker does not necessarily mean that the UK-flagged oil tanker Stena Impero, detained by Iran in the Strait of Hormuz in retaliation for the seizure off Gibraltar, will be freed.

“There is no special relation between the two ships,” the Fars news agency cited Mr Moussavi as saying later.

He said the fate of Stena Impero is with the Iranian court.

“We are glad that our stance about the unlawful and illegal seizure of our tanker has been proven,” Mr Moussavi said. "Regarding the release of the law-breaking British tanker, we have to wait for the court's ruling.

“This tanker has committed two to three nautical violations that are being investigated. We hope that these investigations will finish as soon as possible and a verdict will be issued and if the verdict orders its release, it can continue to sail its path.”

Stena Bulk, which owns the tanker, said it had no indication how the release of the Iranian tanker may have on its vessel and crew.

“We haven’t got any confirmed information at this stage,” a spokesman told Lloyd’s List. “We’re trying to talk to the ship later, and then we might have some information.

“The renamed Grace 1 is now heading for Kalamata and her ETA isn’t until Sunday night, so who knows what can happen between now and then? But that hasn’t got anything to do with Stena.

“Anything that looks like a positive, we must applaud. But for the moment, there is no information at all that any release is imminent.”

The Nautilus union has written to the Iranian government asking that the owners of Stena Impero be granted access to the vessel in order to check on the crew.

Nautilus general secretary Mark Dickinson asked that either Stena Bulk, or a representative of the International Transport Workers’ Federation, be allowed on board.

“Our joint purpose is to ensure the health and well-being of the crew and that this matter is resolved in a timely manner in order to avoid any further stress to the seafarers and their families,” Mr Dickinson said.

Citing the ITF's efforts in securing the release in July of the Iran-flagged vessel Happiness 1, which was being held by Saudi Arabia, Mr Dickinson wrote: “We share the ITF's priority for the welfare of the seafarers regardless where they come from and that they can perform their duties without fear due to geopolitical differences.

“We urge you to convey your support for the ITF's request that the company and/or the ITF be allowed immediate access to the crew and that the issue be resolved peacefully without undue delay.”

Gibraltar’s Supreme Court formally released Adrian Darya 1 after Tehran gave assurances over its destination and the fate of its cargo of oil. The vessel had been seized on July 4 amid concerns it was transporting oil to Syria, in breach of European Union sanctions.

As anticipated, the US sought to seize the ship through forfeiture proceedings, which were lodged on Saturday. But the following day, the Gibraltar government released a statement saying the US request for mutual legal assistance for its Supreme Court to provide “restraining assistance” was refused.

The unilateral US sanctions did not constitute offences in Gibraltar and there were no equivalent sanctions in Europe.

“The EU sanctions regime against Iran — which is applicable in Gibraltar — is much narrower than that applicable in the US,” Gibraltar said in a statement.

The UK government said Iran must abide by the assurances it gave which resulted in the release of the vessel.

“We will not stand by and allow Iran — or anyone — to bypass vital EU sanctions on a regime that has deployed chemical weapons against its own people,’’ a Foreign Office statement said.

“There is no comparison or linkage between Iran’s unacceptable and illegal seizure of, and attacks on, commercial shipping vessels in the Strait of Hormuz and the enforcement of EU Syria sanctions by the Government of Gibraltar,’’ the statement said.

“Freedom of navigation for commercial shipping must be respected and international law upheld.”