Iranian military commanders warned it will be their “duty” to respond in kind following the dramatic swoop yesterday on 300,000-tonne vessel Grace 1. Grace 1 is believed to have been transporting oil from Iran to a refinery in Syria, flaunting international sanctions.  Royal Marines used helicopters and fast attack boats to capture the vessel – flagged to Panama – in a dramatic operation.  Revolutionary Guard commander Mohsen Rezai however threatened to seize a British tanker in response. Taking to Twitter, he said: “If Britain does not release the Iranian oil tanker, it is the Iranian authorities duty to seize a British oil tanker.” Tehran authorities also summonded the British ambassador to answer for the incident which they have branded "piracy".     Royal Marines were hailed for the “brave” and “bold” operation to seize the tanker suspected of carrying oil destined for the cruel regime of Bashar al-Assad.  The marines worked alongside the authorities in Gibraltar to detain the Grace 1, which was believed to be heading to the Banyas refinery in breach of European Union sanctions. Foreign Secretary Jeremy Hunt said the swift action by the authorities on the Rock and the Royal Marines would deny valuable resources to Assad's "murderous regime". The marines were brought in to use their specialist boarding skills to help ensure the Royal Gibraltar Police could gain access to the tanker at sea.    The first marines onto the vessel descended on ropes from a Wildcat helicopter, while others were in rigid inflatable boats – around 16 Royal Gibraltar Police officers were also involved in the mission. The British ambassador to Tehran met with Iranian ministry of foreign affairs officials on Thursday afternoon after being summoned by Iran, it is understood. He was to reiterate the UK's backing of the firm actions taken by Gibraltar in enforcing the sanctions. Iran's foreign ministry spokesman Abbas Mousavi reportedly told Iranian state TV that the ambassador was summoned over the "illegal seizure" of the tanker. Gibraltar's chief minister Fabian Picardo praised the "brave men and women" of the Royal Marines, the Royal Gibraltar Police, HM Customs Gibraltar and the Gibraltar Port Authority for their work in securing the detention of the vessel and its cargo. Mr Hunt congratulated those involved in the "bold move to enforce Syria sanctions".    Mr Picardo said: "In the early hours of this morning, Gibraltar port and law enforcement agencies, assisted by a detachment of Royal Marines, boarded a supertanker carrying crude oil to Syria. "We have detained the vessel and its cargo. This action arose from information giving the Gibraltar government reasonable grounds to believe that the vessel, the Grace 1, was acting in breach of EU sanctions against Syria. "In fact we have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria." The EU has imposed a series of sanctions against Assad's regime in Syria i