oil tanker, a day after a judge in Gibraltar ordered it to be released.
The Grace 1 supertanker, which is carrying 2.1m barrels of oil, was detained on 4 July on suspicion of illegally transporting oil to Syria.
A last-minute legal attempt by the US to keep the tanker detained was rejected by Gibraltar on Thursday.
Two weeks after Grace 1 was detained, on 19 July, Iran seized the British flagged tanker Stena Impero in the Strait of Hormuz.
Although Iran claimed the ship had violated “international maritime rules”, the seizure of Stena Impero was widely believed to be an act of retaliation.
Iran previously called the detention of Grace 1 an “illegal interception”.
Washington claims it can seize Grace 1, and all of the oil on board, based on alleged violations of the International Emergency Economic Powers Act, bank fraud, money laundering, and terrorism statutes.
It has also ordered $995,000 (£818,000) to be seized from an account at an unnamed US bank linked to Paradise Global Trading LLC, which the US claims is associated with businesses that act for Iran’s Revolutionary Guards.
The US designates Iran’s Islamic Revolutionary Guard Corps as a foreign terrorist organisation.
Gibraltar’s government said it had received assurances from Iran that Grace 1 would not sail to countries “subject to European Union sanctions” – that is, Syria.
The British territory’s chief minister Fabian Picardo added: “We have deprived the Assad regime in Syria of more than $140m worth of crude oil.”
After a judge ordered the tanker’s release, Mr Picardo earlier told the BBC that the vessel would be able to leave as soon as the logistics had been figured out: “Could be today, could be tomorrow.”
Neither Britain nor Gibraltar have responded to the US warrant.