(CNN) The United States has unsealed a warrant for the seizure of an Iranian oil tanker anchored off the coast of Gibraltar. It is the latest twist involving the vessel at the center of a standoff between Tehran and the West.

The warrant comes just a day after a judge in Gibraltar said the Grace 1 supertanker -- which is reportedly carrying 2.1 million barrels of oil -- was free to set sail.

The tanker was seized by authorities while passing through Gibraltar's waters in early July on suspicion of taking oil to Syria -- which would have been in breach of European Union sanctions. Gibraltar is a British territory on Spain's southern coast.

Friday's warrant from the US Justice Department appears to be a last-ditch attempt to stop the tanker setting sail, after it failed to stop Gibraltar's Supreme Court approving its release on Thursday.

Gibraltar said it had received assurances from Iran and the owners of the oil that, were the tanker to be released, its cargo would not be taken to Syria.

Iran meanwhile insisted it had made no such commitments that the ship would not go to Syria. "Because from the early hours of the tanker's detention, we announced that Syria was not its destination," said the country's Foreign Ministry spokesman, Seyed Abbas Mousavi, according to the semi-official Tasnim news.

But the US warrant alleges the tanker was part of a scheme involving "illicit shipments to Syria from Iran by the Islamic Revolutionary Guard Corps (IRGC)," which it deems to be a terrorist organization.

Grace 1 was boarded by Royal British Marines and Gibraltarian authorities.

The scheme "involves multiple parties affiliated with the IRGC and furthered by the deceptive voyages of the Grace 1," the US Department of Justice said in a statement.

The warrant says the vessel, all the oil on board and almost $1 million are "subject to forfeiture," due to violation of money laundering and terrorism statutes.

CNN contacted Gibraltar's Supreme Court but had not received a reply at time of publishing.

Act of 'piracy'

Iran has previously criticized the seizure as "illegal" and condemned the operation as "piracy."

Responding to the latest warrant, Iran warned that any delay in getting the Grace 1 tanker moving out of Gibraltar could provide the US with an opportunity for "abuse," Fars News Agency reported.

For now, the tanker remains in Gibraltar waters as it awaits a new captain and a number of crew members, Fars added. The captain of the ship, who is an Indian national, will be replaced by an Iranian, it said.

The Grace 1 supertanker looms among the sea fog off the coast of Gibraltar.

The report said that the tanker had previously started moving and smoke was seen from the chimney of the engine -- but it didn't get far and is still off the coast of Gibraltar.

A lawyer for the tanker told Fars that the vessel needed to be repaired, and one of the reasons for its suspension was to replace some parts.