Gibraltar police said they have arrested the captain and chief officer of the detained Iranian Grace 1 supertanker accused of violating EU sanctions on Syria and have seized documents and electronic devices from the vessel.
British Royal Marines boarded the tanker off Gibraltar and seized it last week on suspicion that it was breaking EU sanctions by taking oil to Syria.
"The investigation is still ongoing and the Grace 1 continues [to be] detained," police said in a statement on Thursday.
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Earlier Thursday Britain said that three Iranian vessels unsuccessfully tried to impede the passage of a British commercial vessel through the Strait of Hormuz.
The Iranian vessels only turned away after receiving "verbal warnings" from a U.K. navy vessel accompanying the commercial vessel British Heritage, the government said in a statement.
The incident followed U.S. President Donald Trump's warning on Wednesday that he would soon "substantially" increase sanctions on Iran as part of a drive to curb Iran's nuclear program and force Tehran to change its regional behavior.
The same day, Trump spoke with Prime Minister Benjamin Netanyahu about national security issues including Iran, the White House said.
"The two leaders discussed cooperation between the United States and Israel in advancing shared national security interests, including efforts to prevent Iran's malign actions in the region," White House spokesman Judd Deere said on Thursday.
Iran's Revolutionary Guards rejected the claim that they tried to stop the British vessel, the semi-official Fars news agency reported.
According to the BBC, the British Heritage tanker was approached as it was sailing from the Persian Gulf into the Strait of Hormuz, near the island of Musa. The island and its territorial waters are disputed between Iran and the United Arab Emirates, but according to the the report, the British vessels never left international waters.
The incident marked the latest escalation of tensions in the Persian Gulf over the 2015 nuclear deal between Iran and world powers, which the Trump administration abandoned last year.
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