Mohsen Rezaee, currently secretary of the Expediency Discernment Council – an advisory body to the Iranian Supreme Leader – tweeted: "If the United Kingdom does not release the Iranian oil tanker, our officials are obliged to retaliate and grab a British oil tanker."
Rezaee was previously commander of the IRGC and it is unclear whether he is currently involved in military decisions.
The British Foreign and Commonwealth Office told CNN that it has no comment at the moment. in response to the tweet.
On Thursday, Gibraltar Prime Minister Fabian Picardo said the authorities had reason to believe that Grace 1 "is carrying their crude oil supply to the Baniyas refinery in Syria".
He added that the refinery "is the property of an institution subject to European Union sanctions against Syria."
While several online Sch iffs followers say the ship was underway Under Panamanian flag, the Panama Maritime Authority (PMA) issued a statement stating that the Grace 1 register was canceled on 29 May after it was informed that the ship might be financing the Terrorism or is involved in it.
"The PMA … has been informed of the international call for tenders for the ship GRACE 1, which indicates that this vessel may be involved in, or associated with, the financing of terrorism or support the destabilizing activities of certain regions, such as Terrorist groups, "said the PMA.
The seizure of ships "illegal," says Iran.
On Thursday, the British Ambassador to Tehran was summoned for "illegally" confiscating the oil tanker in Iran, Iranian Foreign Ministry spokesman Abbas Mousavi tweeted.
The operation was condemned by a high-ranking Iranian foreign minister, who described it as "piracy" on Friday, according to a semi-official news agency Tasnim.
] The official said the UK has no right to "enforce" its or the EU's unilateral sanctions against other nations "in an extraterritorial manner" and demanded the immediate release of the vessel "at the behest of the US."
On Thursday, the UK Ministry of Defense welcomed the so-called "determined action of the Gibraltarian authorities to enforce the EU's Syrian sanctions regime".
While John Bolton, National Security Advisor to US President Donald Trump, also intervened in the tweet, "America and our allies will continue to prevent regimes in Tehran and Damascus from benefiting from this illegal trade," he wrote.
"Every tanker of Iranian crude behaves similarly: it turns off its AIS transponder and back on and off to avoid the persecution," author Michelle Wiese Bockmann of the article said, CNN said Thursday.
"You have deliberately taken measures to avoid the identification of the ship's destination, ship's location and cargo origin."
Samir Madani, co-founder of Tanker Trackers – which oversees ships over satellite and sea data – told CNN that the Grace 1's journey to Africa took two and a half months, a month longer than usual. He added that he did not believe that the ship carried crude oil, but fuel oil from the Iranian Abadan refinery.
"We can also see that it is heavily loaded in the water, its hull 22.5 meters deep, which means that it is 2 million barrels of a very heavy liquid heavier than crude oil," he said. "Not only that, but their previous orders also involved the transfer of fuel oil in Umm Qasr (Iraq) and Khor Fakkan (UAE)."
Madani added that the ship's target was not Syria's Baniyas refinery, as the waters would be too shallow for the ship.
"Instead, we believe the fuel oil was intended for ship-to-ship transfers to other vessels within the Syrian sea borders," Madani said.
Damien Ward of CNN, Samantha Beech, Ruchi Patel, Sarah Dean, Laura Perez Maestro and Mohammed Tawfeeq contributed to the coverage.