Ms. Mandelker said vessels like the Adrian Darya-1 were used to mask illicit sales that fund Iran’s “malign activities and propagate terrorism.” The tanker’s captain, Akhilesh Kumar, was also sanctioned.Tensions over the tanker began on July 4, when the vessel was seized by British marines and port officials in Gibraltar, a semiautonomous British territory. Iran denied allegations that the tanker was transporting oil to Syria, accused British officials of concocting the story at Washington’s behest and threatened to seize a British ship in retaliation.About two weeks later, Iran seized a 30,000-ton British tanker in the Strait of Hormuz, a narrow passage to the Persian Gulf through which about 20 percent of the world’s crude oil travels. Iranian officials said the tanker had “violated three international naval regulations,” including turning off its GPS locator and polluting the water by dumping crude oil residue.