Iran called on Britain on Friday to immediately release an oil tanker that British Royal Marines seized last week on suspicion it was breaking European sanctions by taking oil to Syria, a foreign ministry spokesman told state news agency IRNA.
"This is a dangerous game and has consequences ... the legal pretexts for the capture are not valid ... the release of the tanker is in all countries' interest," the spokesman, Abbas Mousavi, said.
Iran has warned of reciprocal measures if the tanker is not released.
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Meanwhile, Britain said on Friday it would deploy the destroyer HMS Duncan warship to the Gulf to replace HMS Montrose, maintaining a continuous presence there.
"As part of our long standing presence in the Gulf, HMS Duncan is deploying to the region to ensure we maintain a continuous maritime security presence while HMS Montrose comes off task for pre-planned maintenance and crew change over," the government said.
"This will ensure that the U.K. alongside international partners can continue to support freedom of navigation for vessels transiting through this vital shipping.
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Mousavi accused Britain of seizing the tanker under U.S. pressure. "Such illegal measures could increase tensions in the Persian Gulf," he told IRNA.
Gibraltar's Chief Minister Fabian Picardo told parliament on Friday the action to detain Iranian tanker Grace 1 was a decision the territory took on its own and not at the behest of any other state or third party.
"All relevant decisions in respect of this matter were taken only as a direct result of the government of Gibraltar having reasonable grounds to believe the vessel was acting in breach of established EU sanctions against Syria," Picardo said.
"There has been no political request at any time from any government that Gibraltar should act or not act on one basis or another."
The vessel contained 2.1 million barrels of light crude oil, he said.
Britain said on Thursday that three Iranian vessels tried to block a British-owned tanker passing through the Strait of Hormuz, which controls the flow of Middle East oil to the world, but backed off when confronted by a Royal Navy warship.
Iran denied that its vessels had done any such thing.
Tension between Iran and the West has increased a week after Britain seized the tanker and London said the British Heritage, operated by oil company BP, had been approached in the strait between Iran and the Arabian peninsula.
For decades, Shi'ite-led Iran and its U.S.-backed Sunni Gulf Arab rivals have been locked in proxy battles for predominance in the Middle East, from Syria to Yemen.
Washington tightened sanctions from the start of May, ordering all countries and companies to halt imports of Iranian oil or be banished from the global financial system.
In retaliation to Washington's mounting pressure, Iran has decreased its commitments to the nuclear pact, in defiance of a warning by European countries.
The United States, Iran's longtime foe, blames Tehran for a series of attacks on shipping in the world's most important oil artery since mid-May, accusations Tehran rejects.
The capture of the Iranian tanker comes at a time of sharply increased U.S.-Iranian confrontation in recent weeks. Washington has also dispatched extra troops to the region to counter what it describes as Iranian threats.
"Foreign powers should leave the region because Iran and other regional countries are capable of securing the regional security," Mousavi said.
"...Iran has repeatedly expressed its readiness to hold talks with its neighbors to resolve disputes."