*British-flagged Stena Impero was seized after the tanker was involved in an accident; the ship must go through a legal process before it could be released: Tehran
London wants to de-escalate tensions with Tehran, Foreign Secretary Jeremy Hunt said on Saturday following a meeting of the UK's emergencies committee over Iran's seizure of a British-flagged ship in the Gulf.
The meeting "reaffirmed UK desire to de-escalate," Hunt said, adding that the Stena Impero oil tanker was seized in Omani waters "in clear contravention of international law" in "utterly unacceptable" circumstances.
The seizure came hours after a court in the British overseas territory of Gibraltar said it would extend by 30 days the detention of Grace 1, an Iranian tanker seized two weeks ago in an operation aided by British Royal Marines on allegations of breaching EU sanctions against Syria.
Hunt said that having spoken to his Iranian counterpart Mohamed Javad Zarif, Tehran saw the situation as a "tit for tat".
"Nothing could be further from the truth," he said.
"Grace 1 was detained legally in Gibraltarian waters because it was carrying oil, against EU sanctions, to Syria, and that's why the Gibraltarian authorities acted totally with respect to due process and totally within the law.
"Stena Impero was seized in Omani waters in clear contravention of international law. It was then forced to sail into Iran.
"This is totally and utterly unacceptable. It raises very serious questions about the security of British shipping, and indeed international shipping, in the Strait of Hormuz."
Hunt said parliament would be updated tomorrow about what further measures the British government would take.
"Our priority continues to be to find a way to de-escalate the situation," he said.
Hunt said the Stena Impero must be released, and Britain was "very concerned" about the safety of the 23 crew.
Sources said 18 Indians, three Russians, a Latvian and a Filipino were aboard the Swedish-owned ship.
Britain denounced the seizure as a "hostile act", rejecting Tehran's explanation that it had seized the vessel because it had been involved in an accident.
Iran's Revolutionary Guards posted a video online showing speedboats pulling up alongside the Stena Impero tanker, its name clearly visible.
Troops wearing ski masks and carrying machine guns rappelled to its deck from a helicopter, the same tactics used by British Royal Marines to seize an Iranian tanker off the coast of Gibraltar two weeks ago.
Friday's action in the global oil trade's most important waterway has been viewed in the West as a major escalation after three months of confrontation.
British Defence Secretary Penny Mordaunt called the incident a "hostile act". Foreign Secretary Jeremy Hunt said he had expressed "extreme disappointment" by phone to his Iranian counterpart Zarif. Britain also summoned the Iranian charge d'affaires in London.
A spokesman for Iran's Revolutionary Guards, Brigadier-General Ramezan Sharif, said Tehran had seized the ship in the Strait of Hormuz despite the "resistance and interference" of a British warship which had been escorting it. No British warship was visible in the video posted by the Guards.
Iran's Fars news agency said the Guards had taken control of the Stena Impero on Friday after it collided with an Iranian fishing boat whose distress call it ignored.
The vessel, carrying no cargo, was taken to the Iranian port of Bandar Abbas. It would remain there with its 23 crew -- 18 of them Indians -- while the accident was investigated, Iranian news agencies quoted the head of Ports and Maritime Organisation in southern Hormozgan province, Allahmorad Afifipour, as saying.
Zarif told Britain's Hunt that the ship must go through a legal process before it could be released, Iran's ISNA news agency reported.
The strait, between Iran and the Arabian peninsula, is the sole outlet for exports of most Middle Eastern oil, and the seizure sent oil prices sharply higher. The United States, which tightened sanctions against Iran in May with the aim of halting its oil exports altogether, has been warning for months of an Iranian threat to shipping in the strait.
Another oil tanker, the Mesdar, was also boarded by Iranian personnel on Friday and temporarily forced to divert towards Iran, but later was allowed to continue on its route through the strait. Yesterday Algeria's APS news agency said the Mesdar was owned by Algeria's state oil company Sonatrach.
France, Germany and the European Union joined Britain in condemning the seizure.