(Bloomberg) — Iran’s Islamic Revolutionary Guard Corps seized two Greek tankers in Persian Gulf waters over alleged “violations” on Friday, IRGC’s news website Sepah News said in a statement without further details.

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The seizures come days after Tehran said an Iranian-flagged vessel carrying oil was detained off Greek shores and had its cargo confiscated in coordination with US authorities.

The ability of tankers to move freely through the Persian Gulf is essential to the global oil trade. Tankers hauled 20.7 million barrels a day of crude, or a third of world’s trade through the Strait of Hormuz in 2018, according to the Energy Information Administration. The region is also crucial for liquefied natural gas, with more than a quarter of the world’s supply — mostly from Qatar — passing through it annually, according to the EIA.

Greece’s foreign ministry said one of captured ships was Delta Poseidon, which was sailing in international waters 22 nautical miles off Iran’s coast. Armed men also captured the ship’s crew, including two Greek citizens, it said.

Delta Poseidon is a Suezmax ship that has the capacity to carry more than 1 million barrels of crude. The ship was approaching waters off Basrah, Iraq, this week to load its cargo, according to ship tracking data compiled by Bloomberg. After loading supplies Thursday, it signaled its next port of call of Agioi Theodoroi, Greece, and was expected to reach its destination June 10, data show.

A similar incident was reported to have occurred on another Greek-flagged ship off the Iranian coast, the ministry said, adding that Greek authorities had made a strong protest to the Iranian ambassador in Athens.

(Adds information on one of the arrested tankers in fifth paragraph.)

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