The U.S. move late Friday deepens a weeks-long diplomatic dispute between Tehran and Washington. It also comes amid a standoff between the two countries after President Donald Trump withdrew from an international nuclear accord with Tehran and reimposed sanctions. Tensions in the Persian Gulf have been on the rise since.  

The tanker "Grace 1" was seized last month in a British Royal Navy operation off the coast of Gibraltar. Authorities suspected it of violating European Union sanctions on oil shipments to Syria. Its seizure aggravated fears of a conflict in the Persian Gulf, where Iran claims control of the Strait of Hormuz, a strategic waterway for oil shipments.