MADRID (AP) — Authorities in Gibraltar said they intercepted Thursday an Iranian supertanker believed to be breaching European Union sanctions by carrying a shipment of Tehran’s crude oil to war-ravaged Syria. A senior Spanish official said the operation was requested by the United States. 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 currently target 270 people and 70 entities. 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 in a live telephone interview on state television Thursday night. The Gibraltar authorities didn’t confirm the origin of the ship’s cargo but Lloyd’s List, a publication specializing in maritime affairs, reported this week that the Panama-flagged large carrier was laden with Iranian oil. The vessel likely carried just over 2 million barrels of Iranian crude oil, the data firm Refinitv said. Tracking data showed that the tanker made a slow trip around the southern tip of Africa before reaching the Mediterranean, it said. The tanker’s detention comes at a particularly sensitive time 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 recently approved the passage of a carrier group, bombers and fighter jets to the Persian Gulf. In recent days, Iran has broken through the limit the deal put on its stockpile of low-enriched uranium and plans on Sunday to boost its enrichment. Meanwhile, oil tankers near the Strait of Hormuz have been targeted in mysterious attacks as Iranian-backed rebels in Yemen launch bomb-laden drones into Saudi Arabia. The U.S. has rushed thousands of additional troops, an aircraft carrier, B-52 bombers and F-22 fighters to the region, raising fears of a miscalculation sparking a wider conflict. Last month Iran shot down a U.S. surveillance drone, further stoking those fears. Iran, which has provided vital military support to Assad, extended a $3 billion credit line for oil supplies beginning in 2013 but the Iranian aid dwindled as Washington restored tough sanctions. In November, the U.S. Treasury Department added a network of Russian and Iranian companies to its blacklist for shipping oil to Syria and warned of “significant risks” for those violating the sanctions.