An Iranian Revolutionary Guards commander threatened on Friday 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 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.

British Royal Marines abseiled onto the ship off the coast of the British territory on Thursday and seized it. 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 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 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.

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.