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 at the mouth of the Mediterranean Sea after sailing around Africa from Middle East. Iran condemned the “illegal interception” and the Britain’s Foreign Office confirmed that its ambassador to Tehran had been summoned.

A spokesman for the Foreign Office said the ambassador will reiterate Britain’s support for Gibraltar’s seizure of the ship.

In a statement, Britain’s foreign office said “we welcome this firm action by the Gibraltarian authorities, acting to enforce the EU Syria Sanctions regime.”

“We have reason to believe that the ‘Grace 1’ was carrying its shipment of crude oil to the Banyas Refinery in Syria,” Gibraltar’s Chief Minister Fabian Picardo said in a statement.

“With my consent, our port and law enforcement agencies sought the assistance of the Royal Marines in carrying out this operation.”

“That refinery is the property of an entity that is subject to European Union sanctions against Syria.”

“We have detained the vessel and its cargo,” Picardo said.

Picardo said he had “written this morning to the Presidents of the European Commission and Council, setting out the details of the sanctions which we have enforced.”

Spanish Foreign Minister Josep Borrell told reporters the vessel was detained at the request of the United States.

Shipping data reviewed by Reuters suggests it had been loaded with Iranian oil off the coast of Iran, although its documents say the oil is from neighbouring Iraq.

The authorities in Gibraltar made no reference to the source of the oil when they seized it under the authority of European sanctions against Syria that have been in place for years.

A spokesman for British Prime Minister Theresa May welcomed Gibraltar’s move, though the incident could also signal some tensions within Europe.

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

While Europe has banned oil shipments to Syria since 2011, it had never seized a tanker at sea.

“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 US 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 US - that Europe takes sanctions enforcement seriously and that the EU can also respond to Iranian brinkmanship related to ongoing nuclear negotiations.”

Homayoun Falakshahi, Senior Analyst at London-based energy data firm Kpler said the ship had loaded Iranian crude in mid-April from Iran’s export port of Kharg Island.

A maritime intelligence source said the ship may have made the journey around Africa to avoid the Suez Canal, where such a large super-tanker would have had to unload its oil and refill after passing through, exposing its cargo to potential seizure.

The likelihood that the cargo was Iranian drew a link between the incident and a new US effort to halt all global sales of Iranian crude, which Tehran has described as an illegal “economic war” against it.

Iran has long been supplying its allies in Syria with oil despite such sanctions. What is new now is US sanctions on Iran itself. They were imposed last year when President Donald Trump pulled out of an agreement that guaranteed Tehran access to world trade in return for curbs on its nuclear programme.

Those sanctions have been tightened sharply since May, effectively forcing Iran off of mainstream crude markets, making it desperate for alternative ways to sell oil and more reliant on its tanker fleet to store supplies it cannot sell.

European countries opposed Trump’s decision to exit the nuclear deal last year, and they have promised to help Iran find alternative ways to export.

Agencies