Iran demanded Friday that Britain immediately release an oil tanker it has detained in Gibraltar, accusing it of acting at the bidding of the U.S. A senior foreign ministry official "described the U.K. move as unacceptable" in a meeting with British Ambassador Rob Macaire, who had been summoned to hear a formal protest, the ministry said in a statement. He "called for the immediate release of the oil tanker, given that it has been seized at the request of the U.S., based on the information currently available," the statement added.

Iran should seize a British oil tanker if the Iranian tanker detained off Gibraltar is not released immediately, a senior Revolutionary Guards commander said in a tweet on Friday.

British Royal Marines seized the supertanker Grace 1 on Thursday for trying to take oil to Syria in violation of EU sanctions, a dramatic step that drew Tehran's fury and could escalate its confrontation with the West. Iranian Foreign Ministry spokesman Abbas Mousavi said on Thursday that the Iranian oil tanker was carrying crude oil from Iran, according to the state news agency IRNA. The detention of the 330-meter (1,000 feet) Grace 1 comes at a sensitive time in Iran-EU ties as the bloc mulls how to respond to Tehran announcing it is poised to breach the uranium enrichment limit it agreed to in a troubled 2015 nuclear deal. While the EU has banned oil shipments to war-torn Syria, a close ally of Iran, since 2011, it had never seized a tanker at sea. Unlike the United States, Europe does not have broad sanctions against Iran.