The UK government warned vessels not to enter Gulf waters on Saturday, after a British-flagged tanker was seized by Iran's Revolutionary Guards on Friday.
"We remain deeply concerned about Iran's unacceptable actions which represent a clear challenge to international freedom of navigation," a government spokeswoman said following an overnight meeting after a night-time emergency meeting.
"We have advised UK shipping to stay out of the area for an interim period."
The British-flagged tanker which was seized by Iran on Friday evening is now at anchor off the port of Bandar Abbas with all its crew aboard.
The UK said Iran had seized two vessels in the Gulf, but the British owner of one of the tankers, the Liberian-flagged Mesdar, said it had been temporarily boarded by armed personnel, but was allowed to leave "safe and well".
Iran said on Saturday that it had opened an investigation on Saturday into a British-flagged tanker, which had allegedly collided with a fishing vessel in the Strait of Hormuz, a port authority holding the tanker said.
"The investigation into the cause of the accident has been started today by experts at the headquarters of the Hormozgan province port and maritime (organisation)," Allah-Morad Afifipoor, the authority's director-general, told Fars news agency.
Iran's Islamic Revolutionary Guard Corps said on Friday it had seized the Stena Impero in the Strait of Hormuz for breaking "international maritime rules".
Iran's Revolutionary Guards confirmed they had confiscated a British tanker in the strategic Strait of Hormuz for breaking "international maritime rules".
The Stena Impero tanker "was confiscated by the Revolutionary Guards at the request of Hormozgan Ports and Maritime Organisation when passing through the Strait of Hormuz, for failing to respect international maritime rules", the Guards' official website Sepahnews announced.
The tanker "was led to the shore and handed over to the organisation to go through the legal procedure and required investigations", it said.
The Swedish owners of the Stena Impero said the vessel had come under "attack" in the Strait of Hormuz.
Tensions between the UK and Iran have been tense since British marines assisted Gibraltar authorities in seizing an Iranian tanker heading to Syria.
The UK said it would release the tanker if Tehran gave assurances that it would not deliver oil to Syria, which is under sanctions.
A series of attacks on tankers in the Gulf over the past two months have been blamed on Iran, making the waters increasingly volatile.
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