FILE PHOTO: A British Royal Navy patrol vessel guards the oil supertanker Grace 1, that's on suspicion of carrying Iranian crude oil to Syria, as it sits anchored in waters of the British overseas territory of Gibraltar, historically claimed by Spain, July 4, 2019. REUTERS/Jon Nazca

DUBAI (Reuters) - Britain’s seizure of an Iranian oil tanker off Gibraltar last week will not be “unanswered”, Iran’s armed forces chief of staff, Major General Mohammad Bagheri, said on Tuesday, according to the semi-official Tasnim news agency.

“Capture of the Iranian oil tanker based on fabricated excuses ... will not be unanswered and when necessary Tehran will give appropriate answer,” Bagheri said.

British Royal Marines boarded the ship, Grace 1, off the coast of Gibraltar on Thursday and seized it over accusations it was breaking sanctions by taking oil to Syria.

Iran has demanded the immediate release of the oil tanker, while an Iranian Revolutionary Guards commander threatened on Friday to seize a British ship in retaliation.