Two Greek oil tankers filled with crude oil in the Persian Gulf have been seized by Iran’s paramilitary Revolutionary Guard during helicopter raids in Iraq’s Basra oil terminal.

“The Revolutionary Guard’s navy today seized two Greek tankers for violations in Gulf waters,” said the Guard’s statement, quoted by Iranian state news agency IRNA.

It gave no further details and did not say what the ships’ alleged violations were. The actions were an apparent retaliation for Athens assisting the U.S. in seizing Iranian crude oil in the Mediterranean Sea over violating Washington’s crushing sanctions on the Islamic Republic.

Meanwhile, Iran’s semi-official Tasnim news agency warned in a tweet: “There are still 17 other Greek ships in the Persian Gulf that could be seized”.

A US defence official said it appeared the two ships, Prudent Warrior and Delta Poseidon, had come close to, but not into, Iranian territorial waters on Friday. None of the two issued a mayday.

“Armed men then took the crew captive. A similar incident has been reported on another Greek-flagged vessel, that was carrying seven Greek citizens, close to the coast of Iran,” Greece’s Foreign Ministry spokesperson said.

Polembros Shipping in Greece, that is handling the ship said that they are cooperating with the authorities and making every possible effort to address the situation effectively.

Greece’s foreign ministry said it had spoken to the Iranian ambassador in Athens over the “violent taking over of two Greek-flagged ships” in the Persian Gulf warning warning that the seizure would have “particularly negative consequences” in bilateral relations and in Iran’s relations with the European Union, of which Greece is a member.

“These acts, effectively, amount to acts of piracy,” a ministry statement said calling for the immediate release of the vessels and their crews that have not been publicly identified.