British Royal Marines seized an oil tanker in Gibraltar on Thursday that they say was bringing oil to Syria in violation of EU sanctions — reportedly at the request of the United States — in a move that one expert says 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 the Gulf. 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 Gibraltar authorities who seized it did so under the authority of European sanctions against Syria that have been in place for years, with no reference to the source of the oil.

Spain’s caretaker foreign minister, meanwhile, said the tanker was stopped by British authorities after a request from the United States. Josep Borrell told reporters in Madrid that Spain is assessing the implications of the operation because the detention took place in waters it considers its own.

Britain insists Gibraltar is part of the United Kingdom, but Spain argues it is not, and the tanker operation risks offending the Spanish.

“We’re looking into how this [operation] affects our sovereignty,” said Borrell, who was nominated earlier this week to become the EU’s foreign policy chief.

The Spanish claim that the U.S. requested the operation switched attention to whether the tanker was carrying Iranian crude.

The Gibraltar authorities didn’t confirm the origin of the ship’s cargo but Lloyd’s List, a publication specialized in maritime affairs, reported this week that the Panama-flagged large carrier was laden with Iranian oil. Experts were said to have concluded that it carried oil from Iran because the tanker wasn’t sending geographic information while in Iranian waters. According to a UN list, the ship is owned by the Singapore-based Grace Tankers Ltd.

According to the data firm Refinitv, the vessel likely carried just over two million barrels of Iranian crude oil. Tracking data showed the tanker made a slow trip around the southern tip of Africa before reaching the Mediterranean.

Iran later summoned British ambassador Rob Macaire in response to the “illegal interception,” according to a tweet from Iran’s Foreign Ministry spokesperson, Abbas Mousavi.

‘The EU can also respond to Iranian brinkmanship’

The likelihood the cargo was Iranian draws a link between this incident and a new U.S. effort to halt all global sales of Iranian crude, in what Tehran has described as an illegal “economic war” against it.

European countries have tried to stay neutral in that confrontation, which saw the United States call off airstrikes against Iran just minutes before impact last month, and Tehran amass stocks of enriched uranium banned under a nuclear deal.

In a statement, the Gibraltar government said it had reasonable grounds 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,” 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.”

‘This is the first time that the EU has done something so public and so aggressive,’ a sanctions expert said. (Reuters)

The incident appears to be the first in which European authorities have seized a tanker for allegedly violating Syria sanctions, which have been in place since 2011.

“This is the first time that the EU has done something so public and so aggressive. I imagine it was also co-ordinated 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.”

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

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 own tanker fleet to store supplies it cannot sell.

The U.S.-Iranian confrontation has escalated in recent weeks, taking on a military dimension after Washington accused Tehran of attacking tankers in the Gulf and Iran shot down a U.S. drone. Trump ordered air strikes but called them off at the last minute, later saying too many people would have died.

European countries opposed Trump’s decision to exit the nuclear deal last year and have promised to help Iran find alternative ways to export. But they have so far failed to ease the impact of U.S. sanctions, with all major companies halting plans to invest.

U.S. sanctions cut off Iran’s Syrian allies

Iran has said it wants to keep the nuclear deal alive but cannot do so indefinitely unless it receives some of the promised economic benefits. In the past week, it announced it had accumulated more low-enriched uranium than allowed under the deal, and says it will refine uranium to a greater purity than the deal allows from July 7.

By restricting Iran’s ability to move oil around the globe, the U.S. sanctions have choked off Tehran’s Syrian allies.

Government-controlled areas of Syria suffered acute fuel shortages earlier this year resulting from what Syrian President Bashar al-Assad described as an economic siege. In May, Syria received its first foreign oil supplies for six months with the arrival of two shipments including one from Iran, a source familiar with the shipment said at the time.

“We welcome this firm action to enforce EU sanctions against the Syrian regime and commend those Gibraltarian authorities involved in successfully carrying out this morning’s operation,” a spokesperson for British Prime Minister Theresa May said.

“This sends a clear message that violation of the sanctions is unacceptable.”

Iranian crude exports were around 300,000 barrels per day or less in late June, industry sources said, a fraction of the more than 2.5 million bpd Iran shipped in April 2018, the month before Trump withdrew from the nuclear deal.