Iran said on Sunday that the speed of its investigation into a British-flagged tanker depends on the cooperation of its crew, after ignoring calls to hand back the vessel.

The Stena Impero tanker was impounded with its 23 crew members on board at the port of Bandar Abbas after the Islamic Revolutionary Guard Corps seized it on Friday.

The Guards on Saturday released video footage showing a ship with the Stena Impero's markings being surrounded by speedboats before troops in balaclavas descend down a rope from a helicopter onto the vessel.

The Islamic republic has detained the oil tanker on allegations of failing to respond to distress calls and turning off its transponder after hitting a fishing boat.

Its crew is made up of 18 Indians, including the captain, three Russians, a Latvian and a Filipino.

“All of them are in full health, they are on the vessel and the vessel is... anchored in a safe place,” said Allah-Morad Afifipoor, director-general of the Hormozgan province port and maritime authority.

“We are ready to meet their needs. But we have to carry out investigations with regards the vessel,” he told Press TV.

“The investigation depends on the cooperation by the crew members on the vessel, and also our access to the evidence required for us to look into the matter.

“God willing, we will make every effort to gather all the information as soon as possible,” he added.

Earlier, the head of the Ports and Maritime Organisation of Iran in Hormozgan Province had told Iranian state TV that the crew of the seized ship was safe.

“All 23 crew members aboard the ship are safe and in good health in Bandar Abbas port,” Allahmorad Afifipour said.

Stena Bulk, the vessel's Sweden-based owners, said on Saturday they were preparing a formal request to visit the crew, who are from India, Latvia, the Philippines and Russia.

Stena said in a statement that Iran had asked that a formal request be made for the visit.

Britain considering options to respond

Britain is looking into a series of options to respond to Iran, junior defence minister Tobias Ellwood said on Sunday when asked whether London was considering putting sanctions on Tehran.

“Our first and most important responsibility is to make sure that we get a solution to the issue to do with the current ship, make sure other British-flagged ships are safe to operate in these waters and then look at the wider picture,” he told Sky News.

Asked about the possibility of sanctions, he said: “We are going to be looking at a series of options ... We will be speaking with our colleagues, our international allies, to see what can actually be done.”

The Daily Telegraph newspaper had reported a day earlier that Britain, a party to Iran's 2015 multinational nuclear deal, is planning to target Iran with sanctions in the aftermath of the tanker seizure.

The seizure follows weeks of threats from Tehran to retaliate for Britain's seizure of the Iranian tanker Grace 1 near Gibraltar on July 4 which it suspected was violating sanctions on Syria.