British Royal Marines seized the supertanker Grace 1 on Thursday, accusing it of trying to take oil to Syria in violation of European Union sanctions. But this dramatic step has infuriated Iran, threatening to escalate confrontations with the West. Mohsen Rezai, a senior commander in Iran's elite Revolutionary Guards said in a tweet: "If Britain does not release the Iranian oil tanker, it is the (Iranian) authorities duty to seize a British oil tanker.” He added: "Islamic Iran in its 40-year history has never initiated hostilities in any battles but has also never hesitated in responding to bullies.” Tehran had summoned the British ambassador to voice "its very strong objection to the illegal and unacceptable seizure" of its ship. The Iranian Foreign Ministry said the Iranian oil tanker was carrying crude oil from Iran, according to the state news agency IRNA. Gibraltar said it had reasonable grounds to believe the Grace 1 was carrying crude oil to the Baniyas refinery in Syria. The Grace 1 was impounded off the coast of Gibraltar after sailing around Africa - the longer route from the Middle East to the mouth of the Mediterranean Sea The EU has had sanctions in place for nearly a decade that restrict oil sales to Syria. Europe does not have broad sanctions in place against Iran, but the US does, forcing Tehran into mainstream markets to look for unconventional outlets to sell oil. But Spain also reignited its war of words with Britain over sovereignty in Gibraltar following the seizure of the tanker. Acting Foreign Minister Josep Borrell has claimed Gibraltar detained the supertanker Grace 1 after a request by the US to Britain. Mr Borrell, who is set to take over as the EU’s foreign policy chief, said the Spanish Government had been kept informed about the seizure. But he added they would look at whether it violated its territorial water claims. Spain does not recognise Britain’s sovereignty over Gibraltar, which dates all the way back to a 1713 treaty. 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 a breaking story. More to follow.