Image copyright Irib Image caption Crew members on the British-flagged vessel are Indian, Russian, Latvian and Filipino

There is "growing concern" for the crew of a British-flagged tanker seized by Iran in the Gulf, its owners say.

The 23 crew members are confined to the vessel, which is moored off the Iranian port of Bandar Abbas, Stena Bulk and Northern Marine Management said.

The vessel seizure came amid heightened tensions between Iran and the UK.

Tehran said Stena Impero was "violating international maritime rules" at the time but the UK called the incident an example of "state piracy".

Erik Hanell, president and chief executive of Stena Bulk, urged the UK and Iranian governments to "find a swift resolution so our 23 valued seafarers can return to their families and move on from this ordeal".

"We reiterate that there is no evidence of a collision involving the Stena Impero, and at the time of the seizure the vessel was well within the inbound traffic separation scheme and out-with Iranian territorial waters," he said.

"All required navigational equipment, including transponders, was fully functioning, in compliance with maritime regulations."

Tehran says its Revolutionary Guard detained the Stena Impero on 19 July in the key shipping route of the Strait of Hormuz after it collided with a fishing boat.

Last week Stena Bulk said the crew members - who are Indian, Russian, Latvian and Filipino - were safe after speaking to them for the first time.

After the tanker was seized, the Iranian embassy in India told the Reuters news agency that the crew, including 18 Indian nationals, were in "good health".

It came after UK forces helped authorities in Gibraltar seize Iranian tanker Grace 1 earlier this month because they believed it was carrying oil to a Syrian refinery in breach of EU sanctions.

The captain of the tanker told the BBC Royal Marines used "brute force" in detaining his ship but the Ministry of Defence said the seizure complied with "international rules and norms".

The seizure of the tanker sparked a diplomatic crisis between the UK and Iran, which has escalated over the past four weeks.

The UK government has called for Iran to release Stena Impero and its crew, but both sides have ruled out an exchange of the two seized ships.

Relations between Iran and the US have also deteriorated, after the White House tightened sanctions on Tehran following its withdrawal from the 2015 nuclear deal.

UK-Iran tension in the Gulf

Media playback is unsupported on your device Media caption Footage released by Iran's Revolutionary Guard-affiliated Fars news agency appears to show Stena Impero being seized