Iran summoned the British ambassador in Tehran on Thursday after police and British marines seized a Syria-bound oil tanker off Gibraltar, amid suspicions it was operating in breach of EU sanctions.

Iranian Foreign Ministry spokesman Abbas Mousavi condemned on Twitter the "illegal" seizure of the Panama-flagged ship early Thursday.

British Defence Secretary Penny Mordaunt posted on Twitter four photographs apparently showing marines on board the tanker.

One picture showed the stern of the vessel painted with the words "Grace 1, Panama."

"As ever, fantastic work from the @RoyalMarines who used their specialist skills to board a cargo ship in support of a [Gibraltarian] authorities operation to enforce sanctions on Syria," Mordaunt wrote.

Gibraltar's Chief Minister Fabian Picardo said in an earlier statement that the marines and local police had intercepted the Syria-bound oil "super tanker" and "detained the vessel and its cargo."

Shipping register Lloyd's List reported that the tanker is believed to be carrying Iranian oil.

"This action arose from information giving the Gibraltar government reasonable grounds to believe that the vessel, the Grace 1, was acting in breach of European Union sanctions against Syria," Picardo said.

"In fact, we have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas Refinery in Syria," he added.

British Foreign Secretary Jeremy Hunt congratulated Gibraltar and the marines "for this bold move to enforce Syria sanctions."

"Their swift action has denied valuable resources to Asad's [sic] murderous regime," Hunt tweeted, referring to Syria's Bashar al-Assad.

US National Security Advisor John Bolton applauded the move saying in a tweet: "America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade."

Picardo said the Banyas refinery is owned by "an entity that is subject to European Union sanctions against Syria."

He said he had written to the presidents of the European Commission and European Council to inform them of the sanctions enforced by Gibraltar, a British territory on the southern tip of Spain over which Madrid also claims sovereignty.