The US Justice Department has made a last-ditch attempt to block the release of the Iranian supertanker Grace 1 just a day after Gibraltar ruled the detained vessel could be released.

It comes after a six-week international standoff appeared to have come to an end as a judge in Gibraltar ordered its release.

But on Friday night, the US issued the warrant whilst the vessel, which was being investigated for allegedly delivering oil to Syria in breach of EU sanctions, was still anchored off the coast of Gibraltar.

Earlier on Friday, Iranian officials had said it was due to set sail.

According to tracking data, the ship briefly changed its status on Saturday from “at anchor” to “underway” before reverting to “at anchor”.

The head of the company sorting paperwork and procuring for the Grace 1 oil tanker in the British overseas territory said the vessel could be sailing away in the next “24 to 48 hours,” once new crews dispatched to the territory take over command of the ship.

"The vessel is ongoing some logistical changes and requirements that have delayed the departure," Astralship managing director Richard De la Rosa told The Associated Press.

The tanker had first been seized by British marines on July 4 sparking a diplomatic crisis between London and Tehran.

The deadline for its seizure was due to end on Thursday and a bid by the US to further detain it failed.

Then the US Justice Department issued a warrant for its detention accusing it of being 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 (IRGC), a designated foreign terrorist organization," the ministry said.

The warrant says the vessel, all the oil aboard and $995,000 are subject to forfeiture based on violations of the International Emergency Economic Powers Act (IEEPA), and bank fraud, money laundering, and terrorism forfeiture statutes.

The Grace 1 was due to be renamed and switch to the Iranian flag for its onward journey – after Panama removed it from its international boat registry listing – the deputy head of Iran's Ports and Maritime Organisation, Jalil Eslami, told Iranian state television Friday.

But it is believed the ship was awaiting the arrival of a new crew before leaving Gibraltar.

"At the owner's request, the Grace 1 will depart for the Mediterranean after being reflagged under the Islamic Republic of Iran's flag and renamed as Adrian Darya for the voyage," Mr Eslami said, giving further details of the ship's departure.

"The ship was of Russian origin and Panama-flagged and is carrying two million barrels of Iranian oil," he added.

Speaking to the BBC, Gibraltar's First Minister Fabian Picardo said the ship could depart Gibraltar when it's ready.

"She is able to leave as soon as she organises the logistics necessary in order to sail a ship of that size," Mr Picardo said referring to Grace 1.

However, the head of Gibraltar's government added that the ship could still be obstructed in its departure if a US legal bid to stop it succeeds, Reuters reported.

"It could go back to the court absolutely," Mr Picardo said of the US appeal to Gibraltar’s supreme court.

Officials in Gibraltar and Britain have declined to comment on the status of the US appeal.

United States secretary of state Mike Pompeo tweeted on Thursday to warn that mariners aboard the ship could be barred from entering the United States if they continue to crew the tanker.

Mr Pompeo wrote in the tweet that assisting transportation of the oil was “#NotWorthIt”.

At the same time, the captain in charge of the tanker has indicated he wishes to relinquish command of the vessel.

His lawyer has also said repairs are needed on board Grace 1 and this could prevent its departure, the Associated Press reported.

Mixed messages have come from Iran and Gibraltar as to the terms for the release of Grace 1.

In a statement by the government of Gibraltar on Thursday, Mr Picardo said the release had been granted on the basis of written assurances from Tehran that the ship would not be destined for an entity subject to European Union sanctions.

The tanker had originally been stopped on the grounds it was carrying its cargo of crude oil to Baniyas Oil Refinery in Syria.

However, Tehran has said no such guarantees were made and accused Gibraltar of trying to save face.

"Iran has given no assurances over the Grace 1 not going to Syria to secure its release," the state broadcaster's youth website quoted foreign ministry spokesman Abbas Mousavi as saying on Friday.

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

The crisis over Grace 1 has played out against a backdrop of deteriorating relations between Iran, the United States and its Arab allies.

In May 2018 the US withdrew from the Iran nuclear deal, delivering on promises by President Donald Trump to tear up the 2015 agreement that lifted economic sanctions on Tehran in exchange for it committing to give up its nuclear weapons programme.

Mr Trump said repeatedly on the campaign trail and in office that Iran had not stuck to the spirit of the deal as it expanded its sphere of influence and played a destabilising role across the region, backing paramilitary groups in Yemen, Iraq and Syria.

Britain's uneasy diplomatic position between the US and Europe, which has tried to salvage the nuclear deal, over Iran was brought into sharp focus when Iran seized the British-flagged Stena Impero on July 19 in the Strait of Hormuz, 15 days after Grace 1 was boarded by the British off Gibraltar.