It is the authorities' duty to seize a British oil tanker - Rezai

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