Iran summoned the British ambassador in Tehran over the "illegal seizure" of an Iranian oil tanker in Gibraltar on Thursday, Iranian state TV quoted Foreign Ministry spokesperson Abbas Mousavi as saying.

British Royal Marines seized an oil tanker in Gibraltar on Thursday, accused of bringing oil to Syria in violation of EU sanctions, a dramatic step that could escalate confrontation between the West and Iran.

The Grace 1 tanker was impounded in the British territory on the southern tip of Spain after sailing around Africa, the long route from the Middle East to the mouth of the Mediterranean.

The diplomatic gesture lifted any doubt over Iran's ownership of the vessel, which flies a Panama flag and is listed as managed by a company in Singapore. Panama's Maritime Authority said on Thursday that Grace 1 was no longer listed in Panama's international boat registry as of May 29.

U.S. national security adviser John Bolton said the British move was "excellent news."

"America & our allies will continue to prevent regimes in Tehran & Damascus from profiting off this illicit trade," Bolton said on Twitter.

Shipping data reviewed by Reuters suggests the tanker was carrying Iranian oil loaded off the coast of Iran, although its documents say the oil is from neighboring Iraq.

While Europe has banned oil shipments to Syria since 2011, it had never seized a tanker at sea. Unlike the United States, Europe does not have broad sanctions against Iran.

"This is the first time that the EU has done something so public and so aggressive. I imagine it was also coordinated in some manner with the U.S. given that NATO member forces have been involved," said Matthew Oresman, a partner with law firm Pillsbury Winthrop Shaw Pittman who advises firms on sanctions.

"This is likely to have been meant as a signal to Syria and Iran – as well as the U.S. – that Europe takes sanctions enforcement seriously and that the EU can also respond to Iranian brinkmanship related to ongoing nuclear negotiations," he said.

Authorities in Gibraltar made no reference to the source of the oil or the ownership of the ship when they seized it. But Iran's acknowledgment that it owned the ship, and the likelihood that its cargo was also Iranian, drew a link between the incident and a new U.S. effort to halt all global sales of Iranian crude. Iran describes that as an illegal "economic war."

European countries have so far tried to appear neutral in the escalating confrontation between Tehran and Washington, which saw the United States call off air strikes against Iran just minutes before impact last month.

The Gibraltar government said it had reasonable grounds to believe that the Grace 1 was carrying crude oil to the Baniyas refinery in Syria.

"That refinery is the property of an entity that is subject to European Union sanctions against Syria," Gibraltar Chief Minister Fabian Picardo said. "With my consent, our port and law enforcement agencies sought the assistance of the Royal Marines in carrying out this operation."

A spokesperson for British Prime Minister Theresa May welcomed Gibraltar's move.

Spain, which challenges British ownership of Gibraltar, said the action was prompted by a U.S. request to Britain and appeared to have taken place in Spanish waters. Britain's Foreign Office did not respond to a request for comment.

(Cover: Oil supertanker Grace 1, suspected of carrying Iranian crude oil to Syria, is seen near Gibraltar, Spain, July 4, 2019. /Reuters Photo)