MADRID — Authorities in Gibraltar said they intercepted an Iranian supertanker Thursday believed to be breaching European Union sanctions by carrying a shipment of Tehran’s crude oil to war-ravaged Syria.

Iran’s state-run IRNA news agency described the incident as “an illegal seizure of an Iranian oil tanker.”

Gibraltar port and law enforcement agencies, assisted by Britain’s Royal Marines, boarded the Grace 1 early Thursday, authorities on the British overseas territory at the tip of Spain said in a statement.

It added that the vessel was believed to be headed to the Baniyas Refinery in Syria, a government-owned facility under the control of Syrian President Bashar Assad and subject to the EU’s Syrian Sanctions Regime.

The EU and others have imposed sanctions on Assad’s government over its continued crackdown against civilians. They target 270 people and 70 entities.

Spain’s caretaker foreign minister Josep Borrell said the tanker was stopped by British authorities after a request from the United States.

Iran later summoned the British ambassador in Tehran to answer questions about the operation. Iranian Foreign Ministry spokesman Abbas Mousavi said in a tweet that Rob Macaire was summoned over the “illegal interception” of the ship.

Mousavi later called the ship’s seizure “odd and destructive.”

“It can cause an increase in tensions in the region,” he said Thursday night.

In Madrid, Borrell told reporters 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 that 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 this week to become the EU’s foreign policy chief.

Lloyd’s List, a publication specializing in maritime affairs, reported this week that the Panama-flagged large carrier was laden with Iranian oil. According to a U.N. list, the ship is owned by the Singapore-based Grace Tankers Ltd.

The vessel likely carried just over 2 million barrels of Iranian crude oil, the data firm Refinitv said. Tracking data showed the tanker made a slow trip around the southern tip of Africa before reaching the Mediterranean, it said.

The tanker’s detention comes as tensions between the U.S. and Iran grow over the unraveling of a 2015 nuclear deal, which President Donald Trump withdrew from last year. Trump has also slapped sanctions onto Iran and approved the passage of a carrier group, bombers and fighter jets to the Persian Gulf.