The seizure by Iran's Revolutionary Guard comes as tensions remain high between Iran and the West over stalled negotiations regarding its rapidly advancing nuclear program. This is also a break from recent months during which no major incident happened in the Persial Gulf, despite the area having seen a series of attacks since former President Donald Trump unilaterally withdrew America from the accord in 2018.

The Guard issued a statement on its website, accusing the unnamed tankers of unspecified violations.

Greece's Foreign Ministry said it made a strong demarche to the Iranian ambassador in Athens over the “violent taking over of two Greek-flagged ships” in the Persian Gulf.

“These acts effectively amount to acts of piracy,” a ministry statement said.

The ministry called for the immediate release of the vessels and their crews, and said these acts would have “particularly negative consequences” in bilateral relations and in Iran's relations with the European Union, of which Greece is a member.

The ministry's statement said that earlier Friday, an Iranian helicopter landed on the Greek-flagged Delta Poseidon in international waters some 22 nautical miles off the coast of Iran.

“Armed men then took the crew captive,” it said, adding that two Greek nationals were among the crew.

“A similar incident has been reported on another Greek-flagged vessel, that was carrying seven Greek citizens, close to the coast of Iran,” the ministry said.

A Greek official, speaking on condition of anonymity to discuss details of the attack with a journalist, identified the second ship as the Prudent Warrior. Its owner, Polembros Shipping in Greece, earlier said the company was “cooperating with the authorities and making every possible effort to address the situation effectively”.

Greek officials did not identify the nationalities of the other crew onboard the vessels.

A US defence official, speaking on condition of anonymity to discuss intelligence matters, said it appeared the two ships had come close to — but not in — Iranian territorial waters Friday before drifting off into Iranian waters. The ships also had turned off their tracking devices, another red flag, the official said. However, neither had issued a mayday or a call for help, the official said.

Iran had threatened to take “punitive action” earlier Friday over Athens being involved in the U.S. seizure of an Iranian oil tanker in Greek waters.

Iran's Nour News website, close to its security services, made the threat just as shipping news site Lloyd's List said it believed two Greek tankers had been seized in the Persian Gulf. Quoting anonymous industry sources, Lloyd's reported that the two ships had been boarded after Iranian military helicopters approached them on Friday afternoon. It said both tankers had just been loaded with Iraqi crude oil.

The crude oil cargo of an Iranian-flagged tanker that was stopped in Greek waters last month has been seized and is being transferred to another vessel following a request from the United States, a Greek official said Thursday.

The official said following a “judicial intervention by US authorities concerning the ship's cargo” the process is currently underway, at US government expense, for the oil “to be handed over” off the port of Karystos on the Aegean Sea island of Evia.

The official, who asked not to be identified in order to discuss the case, did not provide further details. A Justice Department spokesman in Washington and the US Embassy in Athens declined to comment Thursday.

Iran's seizure on Friday was the latest in a string of hijackings and explosions to roil a region that includes the Strait of Hormuz, the narrow mouth of the Persian Gulf through which a fifth of all traded oil passes.

The US Navy blamed Iran for a series of limpet mine attacks on vessels that damaged tankers in 2019, as well as for a fatal drone attack on an Israeli-linked oil tanker that killed two European crew members in 2021.

Iranian hijackers also stormed and briefly captured a Panama-flagged asphalt tanker off the United Arab Emirates last year, as well as briefly seizing and holding a Vietnamese tanker in November.

Tehran denies carrying out the attacks, but a wider shadow war between Iran and the West has played out in the region's volatile waters. Meanwhile, the Guard is building a massive new support ship near the strategic Strait of Hormuz as it tries to expand its naval presence in waters vital to international energy supplies and beyond, according to satellite photos obtained by The Associated Press.

(With AP inputs)