TEHRAN, Iran — Iranian state media reported Friday that Tehran has demanded the release of a ship seized by Greek authorities in their waters allegedly under pressure by Washington last month.

The foreign ministry summoned the Swiss envoy, who represents the United States in Tehran as they have no embassy there, to lodge a diplomatic protest over the incident. Greek media has described the ship as a tanker carrying Iranian crude oil in breach of international sanctions.

Iran called the move a “violation of international maritime law and international conventions” in its protest, the IRNA news agency reported.

On Wednesday, Iran summoned the Greek charge d’affaires over the matter. A day later, a Greek official said that following a “judicial intervention by U.S. authorities concerning the ship’s cargo” the ship’s oil was being transferred to another vessel off the port of Karystos on the Aegean Sea island of Evia.

The incident comes amid heightened tensions between the U.S. and Iran, which has been under international sanctions since former President Donald Trump withdrew the U.S. from Iran’s 2015 nuclear accord with world powers in 2018.

Previously, the U.S. had seized the cargo of two tankers suspected of transporting Iranian oil as part of an elaborate sanctions-busting scheme involving forged documents and the repainting of a ship’s deck to cloak illegal shipments.

Details of the seizure were contained in a federal civil case unsealed in February after the Greek-managed vessels discharged their valuable cargo, worth about $38 million, in Houston and the Bahamas at the direction of U.S. law enforcement.