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

The crew of a British-flagged tanker that was seized in the Gulf are "safe", the vessel's owner has said after speaking to them for the first time.

The British-flagged Stena Impero and its 23 crew were taken by Iran's Revolutionary Guard on Friday.

The owner, Stena Bulk, made contact on Tuesday with the ship's master who said they were safe and there was good co-operation with the Iranians on board.

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

Iran said it had detained the Stena Impero on 19 July because it collided with a fishing boat, but Stena Bulk has said it has received no evidence of a collision.

The firm said the family members of the crew - who are Indian, Russian, Latvian and Filipino - were being kept updated on the latest developments and being offered its full support.

Its chief executive, Erik Hanell, said he hoped the phone contact was "a first sign that we will soon see more positive progress from the Iranian authorities".

A spokesman for Stena Bulk said the next step would be to try and get somebody on board to check on the crew, but there was no timeline for when they might be repatriated.

A report from Iran's semi-official Tasnim news agency says the UK has sent a mediator to the country to discuss the freeing of the tanker.

However, the BBC has been told by a senior Foreign Office source that they are not aware of any representatives being sent, adding that diplomatic relations are conducted through the UK's ambassador and embassy in Tehran.

On Monday, Foreign Secretary Jeremy Hunt called on Iran to release the tanker and its crew, describing its seizure as "state piracy".

The same day Iranian state media released images which appeared to show cooks preparing meals and crew members being briefed by an Iranian official.

Image copyright AFP PHOTO / FARS NEWS / ABED GHASEMI Image caption An Iranian official was photographed speaking to some of the 23 crew members on the Stena Impero

Meanwhile, Iranian President Hassan Rouhani has praised Iran's Revolutionary Guard for the seizure in the Strait of Hormuz, saying they were "brave" and acted in a "professional" manner.

Mr Rouhani added that the Strait of Hormuz - a key shipping route - was "no place for games" and no country could ignore international rules, his official website reported.

Media playback is unsupported on your device Media caption Iran crisis: A tale of two tankers

Tensions between the UK and Iran deteriorated earlier this month when Royal Marines seized an Iranian tanker near Gibraltar which was suspected of breaking EU sanctions.

In response to the incident, Iran threatened to seize a British oil tanker.

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.