Downing Street has repeated its calls for Iran to immediately release a British-flagged tanker that was seized in the Gulf on Friday. Prime Minister Theresa May has been chairing the government's emergency committee Cobra to receive updates and discuss security in the area. Her official spokesman described the seizure of the ship as "unacceptable and highly escalatory". The foreign secretary is expected to set out next steps to MPs later. It comes amid reports ministers are considering freezing Iranian assets. On Sunday, ministers denied domestic politics meant the government had taken its "eye off the ball". The detainment of the Stena Impero marks escalating tensions between the UK and Iran, coming weeks after Britain helped seize a tanker carrying Iranian oil. What happened?On Friday, the Stena Impero was seized by the Iranian Revolutionary Guard in the key shipping route of the Strait of Hormuz after Tehran said it was "violating international maritime rules". Video released by Iran's Revolutionary Guard-affiliated Fars news agency appeared to show the moment the tanker was raided. A recording emerged of radio exchanges between HMS Montrose and Iranian armed forces vessels moments before the tanker was seized. Iranian vessel can be heard telling the British frigate it wants to inspect the Stena Impero for security reasons. Iran's state-run IRNA news agency said the tanker was captured after it collided with a fishing boat and failed to respond to calls from the smaller craft. Foreign Secretary Jeremy Hunt said it was illegally seized in Omani waters and forced to sail into Bandar Abbas port in Iran. The seizure of the Stena Impero comes two weeks after Royal Marines helped seize Iranian tanker Grace 1 off Gibraltar, because of evidence it was carrying oil to Syria in breach of EU sanctions. Mr Hunt said the Grace 1 was detained legally, but Iran said it was "piracy" and threatened to seize a British oil tanker in retaliation.