Iran has summoned the British ambassador in Tehran over what it calls the "illegal seizure" of an oil tanker in Gibraltar.
Royal Marines took control of the Grace 1, which is accused of transporting oil to Syria in violation of EU sanctions.
If the boat sailed from Iran then it would also have defied a US ban on Iranian oil exports.
Fabian Picardo, chief minister of Gibraltar, said: "We have reason to believe the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria.
"That refinery is the property of an entity that is subject to European Union sanctions against Syria."
He added: "Be assured that Gibraltar remains safe, secure and committed to the international rules-based legal order."
British foreign secretary Jeremy Hunt congratulated Gibraltar for the move.
Congratulations to Gibraltar & @RoyalMarines for this bold move to enforce Syria sanctions. Their swift action has denied valuable resources to Asad’s murderous regime. https://t.co/XP5cq5bsVt
— Jeremy Hunt (@Jeremy_Hunt) July 4, 2019
The Reuters news agency reports that Iranian officials have summoned the UK ambassador to Iran, Robert Macaire, to inform him of “its very strong objection to the illegal and unacceptable seizure”.
Spanish foreign minister Josep Borrell, meanwhile, said his country was looking into the seizure of the ship as it appears to have happened in Spanish waters.
The latest development comes amid the ongoing tensions between Iran and the US in recent weeks, following a series of incidents - including the downing of a US drone by Iranian forces.
