Oil supertanker Grace 1 on suspicion of being carrying Iranian crude oil to Syria is seen near Gibraltar, Spain in this picture obtained from social media, July 4, 2019. Photo: Stephen McHugh via REUTERS.

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.

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.

Mousavi accused Britain of seizing the tanker under US pressure. “Such illegal measures could increase tensions in the Persian Gulf,” he told IRNA.

For decades, Shi’ite-led Iran and its US-backed Sunni Gulf Arab rivals have been locked in proxy battles for predominance in the Middle East, from Syria to Yemen.

Britain is among European parties to Iran’s 2015 nuclear deal with six major powers, which President Donald Trump pulled out of last year, reimposing sanctions on Tehran.

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 US-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.”