“We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas Refinery in Syria,” Fabian Picardo, the chief minister of Gibraltar, said in a statement. “That refinery is the property of an entity that is subject to European Union sanctions against Syria.”

Spain’s acting foreign minister, Josep Borrell, told Spanish news media that the oil tanker had been seized following “a request from the United States to the United Kingdom.” He said that Spain had been kept informed about the seizure, but offered no details to support his assertion about an American request.

Although it is not illegal under international law to buy or ship Iranian oil or related products, foreign companies that do so risk punishment by the United States. Iran has been largely cut off from European markets by the American sanctions, which were imposed by the Trump administration after it withdrew the United States from the nuclear accord.

The European parties to the nuclear accord, including Britain, have tried to preserve the fraying deal, which had promised economic relief in exchange for limits to Iran’s nuclear program. But Tehran exceeded a critical limit of the accord on Monday, and threatened to breach another as early as Sunday, after weeks of escalating tensions with the United States and months of increasingly dire economic conditions in Iran.



Some of the world’s shipping fleets have defied the sanctions to continue doing business with Iran. Companies that monitor international shipping say that the Grace 1 turned off its electronic tracking devices as it sailed into Iranian waters, then turned them on again after leaving — a tactic often used to evade the sanctions, though ships can still be followed through satellite photography.