The Grace 1 supertanker was held by British Royal Marines in July on suspicion of illegally transporting oil to Syria.
Authorities in the US claim they can seize the ship and the 2.1m barrels of oil it is carrying over alleged violations of terrorism statutes and the International Emergency Economic Powers Act. But officials in Gibraltar have already allowed the Grace 1 to leave.
Two weeks after the vessel was detained, Iran seized the British-flagged oil tanker Stena Impero in the Strait of Hormuz in apparent retaliation.
A last-minute legal attempt by the US to keep the Grace 1 detained was rejected by Gibraltar on Thursday.
The following day, the US federal court issued a warrant calling for the tanker and oil on board to be seized.
The warrant was issued by the US district court for the District of Columbia and addressed to “the United States Marshal’s Service and/or any other duly authorised law enforcement officer”.
It has also ordered the seizure of $995,000 from an account at an unnamed US bank linked to Paradise Global Trading LLC, an Iranian company.
"A network of front companies allegedly laundered millions of dollars in support of such shipments," federal prosecutor Jessie Liu said in a statement.
She added the parties involved were linked with Iran's Islamic Revolutionary Guard Corps, which the US regards as a foreign terrorist organisation.
Gibraltar's chief minister, Fabian Picardo, said the warrant would be subject to the jurisdiction of Gibraltar's Supreme Court.
The Grace 1 shifted its position on Friday, but its anchor was still down and it was unclear if it was ready to set sail soon. The ship had its name removed and was no longer flying a Panama flag.
The Gibraltar Chronicle reported that the Grace 1 was unlikely to sail before Sunday, citing an unnamed source who added that it was waiting for six new crew members including a captain to arrive.
It is to be renamed and will fly an Iranian flag for its onward journey, the deputy head of Iran’s ports and maritime organisation, Jalil Eslami, told Iranian state television on Friday.
Additional reporting by agencies