Mr Borrell said, according to the Europa Press news agency: "We're looking at which way this affects our sovereignty, in as far as it took place in waters whose sovereignty we understand to belong to Spain".
"This is the first time that the European Union has done something so public and so aggressive".
The detention of the 330 metre (1,000 feet) Grace 1 vessel comes at a sensitive time in Iran-EU ties as the bloc mulls how to respond to Tehran announcing it will breach the maximum uranium enrichment level it agreed in a 2015 nuclear deal. The distance from Iran to Syria, sailing around Africa, is about 23,300 kilometers (14,500 miles), compared with just 6,600 kilometers via the Red Sea and Suez Canal.
"Gibraltar, he added, had reasonable grounds to believe that the tanker was carrying its crude oil shipment to the Banyas refinery in Syria, which is the property of an entity that is subject to European Union sanctions against Syria". The Royal Marines rappelled from a helicopter onto the deck while approximately 16 police officers from Gibraltar were also involved in the operation.
Later on Thursday, acting Spanish Foreign Minister Josep Borrell said the supertanker was impounded after a request by the United States to Britain.
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He said the interception was made exclusively on the breach of European sanctions and information that the cargo was destined for Syria, rather than any breach of unilateral U.S. sanctions.
The tanker's detention comes at a particularly sensitive time as tensions between the United States and Iran grow over the unraveling of a 2015 nuclear deal, which President Donald Trump withdrew the USA from previous year.
Iran has regularly supplied Syria with crude oil since the country descended into civil war in 2011.
The United States imposed heavy sanctions on Iran, particularly affecting its oil exports and global banking, after the United States withdrew from the nuclear deal with Tehran in May 2018. They now target 270 people and 70 entities. The shipment comes days after the European Union introduced Instex, a financial trading instrument that circumvents U.S. sanctions, Lloyds List said.
European countries opposed Trump's decision to exit the nuclear deal a year ago, and they have promised to help Iran find alternative ways to export, but with little success so far.
Iran has said it wants to keep the nuclear deal alive but can not do so indefinitely unless it receives promised economic benefits.
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By restricting Iran's ability to move oil, US sanctions have choked off Tehran's Syrian allies, causing fuel shortages in government-controlled areas.
The Grace 1 was documented as loading fuel oil in Iraq in December, although the Iraqi port did not list it as being in port and its tracking system was switched off.
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.
The British overseas territory at the tip of Spain said the port and law enforcement agencies, assisted by the Royal Marines, boarded the Grace 1 early Thursday.
The tanker reappeared near Iran's port of Bandar Assaluyeh fully loaded.
Earlier, British Royal Marines and officials from the British colony of Gibraltar detained a vessel suspected of transporting oil to Syria in violation of European Union sanctions. The European Commission-operated Equasis website lists the shipmanager as Singapore-based Iships Management.
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