Britain should be "scared" about Tehran's possible retaliation over the capture of an Iranian supertanker by Royal Marines in Gibraltar, Fars news agency on Saturday cited an Iranian cleric as saying.
The Grace 1 tanker was impounded on Thursday in the British territory on the southern tip of Spain after sailing around Africa — the long route from the Middle East to the mouth of the Mediterranean.
"I am openly saying that Britain should be scared of Iran's retaliatory measures over the illegal seizure of the Iranian oil tanker," said Mohammad Ali Mousavi Jazayeri, a member of the powerful clerical body the Assembly of Experts.
"We have shown that we will never remain silent against bullying ... As we gave a staunch response to the American drone, the appropriate response to this illegal capture (of the tanker) will be given by Iran as well," he added.
An Iranian Revolutionary Guard commander threatened on Friday to seize a British ship in retaliation for the capture of an Iranian supertanker by the Royal Marines in Gibraltar.
"If Britain does not release the Iranian oil tanker, it is the authorities' duty to seize a British oil tanker," Mohsen Rezai said on Twitter.
The Gibraltar government said the crew onboard 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.
A government spokesman said the 28-member crew, who have remained on board the supertanker, were mainly Indians with some Pakistanis and Ukrainians.
Police and customs officials remained on board the vessel to carry out their investigation, but the Royal Marines were no longer present.
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 seized a giant Iranian oil tanker in Gibraltar on Thursday suspected of taking oil to Syria in violation of EU sanctions. The move sparked Tehran's fury and could escalate its confrontation with the West.
Iran's Foreign Ministry summoned the British ambassador to voice "its very strong objection to the illegal and unacceptable seizure" of its ship.
US national security adviser John Bolton said the British move was "excellent news."
While Europe banned oil shipments to Syria in 2011, it has never seized a tanker at sea. Unlike the United States, Europe does not have broad sanctions against Iran.
EU 'taking sanctions seriously'
"This is the first time that the EU has done something so public and so aggressive. I imagine it was also coordinated in some manner with the US given that NATO member forces have been involved," said Matthew Oresman, a partner with law firm Pillsbury Winthrop Shaw Pittman, which advises firms on sanctions.
"This is likely to have been meant as a signal to Syria and Iran — as well as the US — that Europe takes sanctions enforcement seriously and that the EU can also respond to Iranian brinkmanship related to ongoing nuclear negotiations," he said.
Authorities in Gibraltar made no reference to the source of the oil or the ownership of the ship when they seized it.
But Iran's acknowledgement that it owned the ship, and the likelihood that its cargo was also Iranian, linked the incident and a new US effort to halt all global sales of Iranian crude. Iran described the move as an illegal "economic war."
European countries have so far tried to remain neutral in the escalating confrontation between Tehran and Washington, which saw the United States call off airstrikes against Iran just minutes before impact last month, and Tehran amass stocks of enriched uranium banned under a 2015 nuclear deal.
The Gibraltar government said it had reasonable grounds to believe that the Grace 1 was carrying crude oil to the Baniyas refinery in Syria.
"That refinery is the property of an entity that is subject to European Union sanctions against Syria," Gibraltar Chief Minister Fabian Picardo said. "With my consent, our port and law enforcement agencies sought the assistance of the Royal Marines in carrying out this operation."
A spokesman for British Prime Minister Theresa May welcomed Gibraltar's move.
US-Iran confrontation escalating over nuclear deal
Spain, which challenges British ownership of Gibraltar, said the action was prompted by a US request to Britain and appeared to have taken place in Spanish waters. Britain's Foreign Office did not respond to a request for comment.
Iran has long been supplying its allies in Syria with oil despite sanctions against Syria. What is new are US sanctions on Iran itself, imposed last year when President Donald Trump pulled out of an agreement that had guaranteed Tehran access to world trade in return for curbs on its nuclear programme.
These US sanctions have been tightened since May, effectively forcing Iran from mainstream oil markets and making it desperate for alternative customers.
The US-Iranian confrontation has escalated in recent weeks, taking on a military dimension after Washington accused Tehran of attacking tankers in the Gulf and Iran shot down a US drone. Trump ordered airstrikes but called them off at the last minute, later saying too many people would have died.
European countries opposed Trump's decision to exit the nuclear deal last year, and they have promised to help Iran find alternative ways to export, but with little success so far.