A last-minute US warrant to seize an Iranian tanker preparing to leave Gibraltar after weeks of detention cast doubt over its departure on Saturday, prolonging a diplomatic spat between

Tehran, London and Washington.

The US Justice Department alleged the ship was part of a scheme "to unlawfully access the US financial system to support illicit shipments to Syria from Iran by the Islamic Revolutionary Guard Corps," which Washington has designated a foreign terrorist organisation.

There was no immediate comment from Britain or Gibraltar, its overseas territory, over whether they would act on the warrant.

Iran subsequently detained the British-flagged tanker Stena Impero in what was seen as a tit-for-tat move.

On Thursday, Gibraltar's Supreme Court ordered the tanker released after the British overseas territory said it had received assurances from Iran that the Grace 1 would not head to any country subject to European Union sanctions.

Preparations are underway for the ship to set sail, with a new crew set to arrive to pilot the tanker and its 2.1 million barrels of oil.

But in a last-minute twist on Friday, the United States, which is at loggerheads with Iran, issued the warrant.

The warrant says the vessel and all the oil aboard are subject to forfeiture based on violations of a US law, the International Emergency Economic Powers Act, which revolves around any unusual or extraordinary threat from overseas to the national security, foreign policy or economy of the United States.

It also cites violations to bank fraud, money laundering and terrorism statutes.

- Ship released on Thursday -

The US move comes after it tried -- and failed -- to block the tanker's release on Thursday.

The July 4 seizure came amid surging tensions in the Gulf after several alleged Iranian attacks on smaller tankers.

The US -- citing Tehran's threat to American allies -- expanded its military presence in the region with a new aircraft carrier task force, missile batteries and strategic bombers.

Iran called the detention of the Grace 1 an "illegal interception" staged by the United States, while Washington cheered it as "excellent news."

Ties between Tehran and Washington are at a low point since US President Donald Trump withdrew last year from a landmark 2015 nuclear deal between major powers and Iran, reimposing crippling unilateral sanctions.

- New crew arrive -

On Friday, Iranian officials said the tanker was preparing to set sail under an Iranian flag and would be renamed the Adrian Darya for the voyage.

Iranian Foreign Minister Mohammad Javad Zarif tweeted that the US attempt at "piracy" had failed, saying it showed Washington's "contempt for the law."

A source close to the case, who refused to be named, told the Gibraltar Chronicle daily that the ship was awaiting the arrival of a new crew before it would leave Gibraltar, and this was "unlikely" to be before Sunday.

The US State Department has threatened to issue a visa ban on anyone working on the ship.

Iran denied Friday it had made any promises about the ship's destination to secure the release.

"Iran has given no assurances over the Grace 1 not going to Syria to secure its release," a state media website quoted foreign ministry spokesman Abbas Mousavi as saying.

"The tanker's destination was not Syria... and even if it was, it did not concern anyone else."

But Gibraltar insisted it was.

"We have deprived the Assad regime in Syria of more than $140 million worth of crude oil," Gibraltar Chief Minister Fabian Picardo said.afp