The Iranian Revolutionary Guards threatened on Friday Britain that Tehran will capture one of its vessels in retaliation for the capture of an Iranian supertanker by Royal Marines in Gibraltar. “If Britain does not release the Iranian oil tanker, it is the authorities’ duty to seize a British oil tanker,” said Mohsen Rezai, a Guards commander, said on Twitter. The Gibraltar government said the crew on board the supertanker Grace 1 were being interviewed as witnesses, not criminal suspects, in an effort to establish the nature of the cargo and its ultimate destination. US President Donald Trump, while not specifically mentioning the supertanker incident, repeated a warning to Tehran: “We’ll see what happens with Iran. Iran has to be very, very careful,” he told reporters at the White House. British Royal Marines boarded the ship off the coast of the British territory on Thursday and seized it over accusations it was breaking sanctions by taking oil to Syria. They landed a helicopter on the moving vessel in pitch darkness. "We have reason to believe that the Grace 1 was carrying its shipment of crude oil to the Banyas refinery in Syria," Gibraltars Chief Minister Fabian Picardo said. In a statement, Britains Foreign Office said: "We welcome this firm action by the Gibraltarian authorities, acting to enforce the EU Syria sanctions regime". Spanish Foreign Minister Josep Borrell told reporters the vessel was detained at the request of the United States. The move escalates a confrontation between Iran and the West just weeks after the United States called off airstrikes on Iran minutes before impact, and draws Washington’s close ally into a crisis in which European powers had striven to appear neutral. Tehran summoned the British ambassador on Thursday to voice “its very strong objection to the illegal and unacceptable seizure” of its ship, a move that also eliminated doubt about the ownership of the vessel. Foreign Ministry spokesman Abbas Mousavi said the crude oil cargo was from Iran. The ship’s paperwork had said the oil was from neighboring Iraq, but tracking data reviewed by Reuters suggested it had loaded at an Iranian port. European Union sanctions against war-torn Syria have been in force since late 2011. The 28-member bloc has imposed sanctions on Syrian officials including government ministers over their role in the "violent repression" of civilians. The tankers detention comes just days after Iran announced it would exceed the uranium enrichment limit set under the 2015 nuclear deal. Tehran took the action in response to Washington abandoning the deal last year and hitting Irans crucial oil exports and financial transactions with biting sanctions. White House National Security Advisor John Bolton, a champion of the hawkish policy towards Tehran, applauded the interception of the supertanker. "Excellent news: UK has detained the supertanker Grace I laden with Iranian oil bound for Syria in violation of EU sanctions," Bolton tweeted. The Grace 1 was impounded in the British territory on the southern tip of Spain after sailing the long way around Africa from the Middle East to the mouth of the Mediterranean, a route that demonstrates the unusual steps Iran appears to be taking to try to keep some exports flowing. Shipping experts say it may have been avoiding the more direct route through the Suez Canal, where a big tanker would typically be required to unload part of its cargo into a pipeline to cross, potentially exposing it to seizure. Olivier Dorgans, an economic sanctions expert at Hughes Hubbard & Reed law firm in Paris, said the British move appeared intended to send a warning to the Iranians that if they pushed on with their nuclear breaches, European countries would act. “This was done for political effect. The British are warning the Iranians,” he said according to Reuters.