Iran called on Friday Britain to immediately release an oil tanker than it impounded last week as Gibraltar said the vessel was suspected of breaching EU sanctions against Syria. “This is a dangerous game and has consequences ... the legal pretexts for the capture are not valid ... the release of the tanker is in all countries’ interest,” Iranian foreign ministry spokesman, Abbas Mousavi, said. Tehran has warned of reciprocal measures if the tanker is not released. Gibraltar said its action to detain the Grace 1 was a decision it took on its own and not at the behest of any other state or third party. “All relevant decisions in respect of this matter were taken only as a direct result of the government of Gibraltar having reasonable grounds to believe the vessel was acting in breach of established EU sanctions against Syria,” the territory’s chief minister, Fabian Picardo, told parliament. “There has been no political request at any time from any government that Gibraltar should act or not act on one basis or another.” The vessel contained 2.1 million barrels of light crude oil, he said. Britain said on Thursday that three Iranian vessels tried to block a British-owned tanker passing through the Strait of Hormuz, which controls the flow of Middle East oil to the world, but backed off when confronted by a Royal Navy warship. Iran denied that its vessels had done any such thing. Commenting on the incident, UN Secretary-General Antonio Guterres again called for "maximum restraint" in the Arabian Gulf region and warning all parties that a new confrontation "would be a catastrophe." UN deputy spokesman Farhan Haq reiterated Thursday the UN chiefs plea to avoid any escalation. Responding to a question on whether the secretary-general supports US and other calls to safeguard ships passing through the Gulf, he reiterated Guterres call to avoid escalation. "We want, of course, for everyone to allow for the freedom of movement of vessels and were hopeful that they will abide by that," Haq said. Tension between Iran and the West has increased a week after Britain seized the tanker and London said the British Heritage, operated by oil company BP, had been approached in the strait between Iran and the Arabian peninsula. Mousavi accused Britain of seizing the tanker under US pressure. “Such illegal measures could increase tensions in the Gulf,” he told IRNA. Police in the British territory of Gibraltar said on Thursday they had arrested the captain and chief officer of the Grace 1 supertanker accused of violating EU sanctions on Syria and seized documents and electronic devices from the ship. Britain is among European parties to Iran’s 2015 nuclear deal with six major powers, which President Donald Trump pulled out of it last year, reimposing sanctions on Tehran. Washington sharply tightened sanctions against Iran since May with the aim of bringing its oil exports to zero. Iran responded by stepping up production of enriched uranium beyond limits in the nuclear deal. Britain’s seizure of the Grace 1 changed the diplomatic calculus after weeks in which Washington’s European allies have strained to appear neutral. They disagreed with Trump’s decision to quit the nuclear pact last year. Although EU states have not followed Washington in imposing sanctions on Iran, they have sanctions in place that forbid selling oil to Iran’s ally Syria.