The Grace 1 was seized by British Royal Marines at the western mouth of the Mediterranean on July 4 on suspicion of violating European Union sanctions by taking oil to Syria.
Gibraltar lifted the detention order on Thursday after the British territory’s chief minister said he had secured written assurances from Tehran that the cargo would not go to Syria.
But with the vessel and its 2.1 million barrels of oil free to leave, the United States launched a separate legal appeal to impound the ship on the grounds that it had links to Iran’s Islamic Revolutionary Guard Corps (IRGC), which it designates as a terrorist organization.