The Stena Impero vessel was impounded at Iran's Bandar Abbas port. (File)

The British-flagged oil tanker Stena Impero, which had been held off the Iranian port of Bandar Abbas for more than two months, set sail Friday, an Iranian shipping organisation said.

The ship's seizure was widely seen as a tit-for-tat move after authorities in the British overseas territory of Gibraltar detained an Iranian tanker on suspicion it was shipping oil to Syria in breach of EU sanctions.

Tehran repeatedly denied the cases were related but a Gibraltar last month ordered the Iranian tanker's release despite an 11th-hour US legal bid to keep it in detention.

"The Stena Impero started sailing from the mooring towards the Persian Gulf's international waters as of 9:00 am (0530 GMT) today," Hormozgan province's maritime organisation said on its website.

"Despite the vessel's clearance, its legal case is still open in Iran's courts," it added.

The tanker's captain and crew have also "given a written, official statement that they have no claims."

Iran's Revolutionary Guards seized the tanker in the Strait of Hormuz on July 19 after surrounding the vessel with attack boats and rappelling onto its deck.

It was impounded off the port of Bandar Abbas for allegedly failing to respond to distress calls and turning off its transponder after hitting a fishing boat.

Seven of its 23 crew members were released on September 4.