WASHINGTON — The Treasury Department on Friday imposed sanctions on an Iranian oil tanker at the center of a power struggle between the United States and Iran, a move that threatened to further escalate tensions between the two countries.

The tanker — once known as the Grace 1, and renamed the Adrian Darya-1 — had been detained in Gibraltar for weeks on suspicion of violating European Union sanctions by trying to transport oil to Syria. It was released two weeks ago, despite a last-ditch effort by American officials to have the tanker turned over to them.

The tanker is believed to be transporting more than two million barrels of Iranian crude oil, testing American sanctions intended to limit the country’s ability to sell oil.

The ship’s reported destination changed repeatedly throughout Friday. First, it was said to be bound for Turkey, then for Lebanon, only for Lebanese officials to say they did not believe it was coming there.