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 (July 9), 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 Teheran will give appropriate answer," Maj-Gen Bagheri said.

British Royal Marines boarded the ship, Grace 1, off the coast of Gibraltar last 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 last Friday to seize a British ship in retaliation.