An Iranian Revolutionary Guards commander has threatened to seize a British ship in retaliation for the capture of an Iranian supertanker in Gibraltar by Royal Marines.

“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 on Friday it had obtained a court order extending the detention of the Grace I supertanker by 14 days because there were grounds to believe it was breaking sanctions by taking oil to Syria.

The government said earlier the crew were being interviewed as witnesses, not criminal suspects, in an effort to establish the nature of the cargo and its ultimate destination.

British Royal Marines abseiled onto 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 US called off air strikes minutes before impact.

It also 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 neighbouring Iraq, but tracking data reviewed by Reuters suggested it had loaded at an Iranian port.

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.

The route demonstrates the unusual steps Iran appears to be taking to try to keep some exports flowing.

The Gibraltar 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.

Shipping experts say the Grace 1 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.