LONDON/DUBAI (Reuters) - An Iranian Revolutionary Guards commander threatened on Friday to seize a British ship in retaliation for the capture of an Iranian supertanker by Royal Marines in Gibraltar.

A helicopter hovers near the Iranian oil tanker Grace 1 off Gibraltar, according to Britain’s Ministry of Defence, in a night vision photograph released July 4, 2019. British Royal Marines seized the Grace 1 tanker on Thursday for trying to take oil to Syria in violation of European Union sanctions. MoD/Handout via REUTERS

“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 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.

(Graphic: Oil supertanker detained in Gibraltar - tmsnrt.rs/2Yv07qx )

U.S. 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.

The move escalates a confrontation between Iran and the West just weeks after the United States called off air strikes on Iran minutes before impact, and draws Washington’s close ally into a crisis in which European powers had striven to appear neutral.

A U.S. State Department spokeswoman said, “We welcome international partners’ resolve in upholding and enforcing these sanctions.”

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.

THIN LINE

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 countries have walked a thin line since last year when the United States ignored their pleas and pulled out of a pact between Iran and world powers that gave Tehran access to global trade in return for curbs on its nuclear program.

Over the past two months, Washington has sharply tightened sanctions against Tehran with the aim of halting its oil exports altogether. The moves have largely driven Iran from mainstream markets and forced it to find unconventional ways to sell crude.

The confrontation has taken on a military dimension in recent weeks, with Washington accusing Iran of attacking ships in the Gulf and Iran shooting down a U.S. drone. Trump ordered, then canceled, retaliatory strikes.

With nuclear diplomacy at the heart of the crisis, Iran announced this week it had amassed more fissile material than allowed under its deal, and said it would purify uranium to a higher degree than permitted from July 7.

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.

“WARNING THE IRANIANS”

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The Gibraltar 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.

While the European Union has not followed the United States in imposing broad sanctions against Iran, it has had measures in place since 2011 that prohibit sales of oil to Syria.

Gibraltar said on Friday it had obtained an order extending the detention of the supertanker by 14 days because there were grounds to believe it was breaking sanctions by taking crude oil to Syria.

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.”