Iran's paramilitary Revolutionary Guard has seized two Greek oil tankers in helicopter-launched raids in the Persian Gulf.

"The Revolutionary Guard's navy today seized two Greek tankers for violations in Gulf waters," said a 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.

Greece's Foreign Ministry said an Iranian navy helicopter landed on Greek-flagged vessel, the Delta Poseidon, which was sailing in international waters, 22 nautical miles from the Iranian shore, and took the crew hostage, among them two Greek citizens.

"Armed men then took the crew captive," it said.

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

A Greek official identified the second ship as the Prudent Warrior.

Its manager, 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 two vessels.

Both vessels had come from Iraq's Basra oil terminal, loaded with crude, according to tracking data from MarineTraffic.com.

A US defence official said it appeared the two ships had come close to — but not into — Iranian territorial waters on Friday.

After the hijacking, the ships drifted into Iranian waters.

The ships also had turned off their tracking devices, another red flag, the official said.

However, neither had issued a mayday nor a call for help, the official said.

Greece calls tanker seizure 'piracy'

The action appeared to be retaliation for Athens' assistance in the US seizure of crude oil from an Iranian-flagged tanker this week in the Mediterranean Sea over violating Washington's crushing sanctions on the Islamic Republic.

These latest raids mark the first major incident at sea in months as tensions remain high between Iran and the West over its tattered nuclear deal with world powers.

As Tehran enriches more uranium — closer to weapons-grade levels than ever before — worries mount that negotiators won't find a way back to the accord, raising the risk of a wider war.

The Guard issued a statement announcing the seizures, accusing the tankers of unspecified violations.

Nour News — a website close to Iran's Supreme National Security Council — just a short time earlier warned that Tehran planned to take "punitive action" over Greece assisting the US in seizing oil days earlier from the Lana, an Iranian-flagged tanker.

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.

"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, 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.

Threats of further action from Iran

Greek authorities last month impounded the Iranian-flagged Pegas, with 19 Russian crew members on board, near the coast of the southern island of Evia due to European Union sanctions.

The US later confiscated the Iranian oil cargo held onboard and plans to send it to the US on another vessel.

Afterwards, the Pegas was released.

The Pegas was among five vessels designated by Washington on February 22 — two days before Russia's invasion of Ukraine — for sanctions against Promsvyazbank, a bank viewed as critical to Russia's defence sector.

It was unclear whether the cargo was impounded because it was Iranian oil or due to the sanctions on the tanker over its Russian links.

Iran and Russia face separate US sanctions.

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.

Tanker seizures have been a part of that since 2019, when Iran seized the British-flagged Stena Impero after the United Kingdom detained an Iranian oil tanker off Gibraltar.

Iran released the tanker months later when London also released the Iranian vessel.

Iran last year also seized and held a South Korean-flagged tanker for months amid a dispute over billions of dollars of frozen assets that Seoul holds.

Underlining that threat, 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."

Meanwhile, the Guard is building a massive new support ship near the 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.

ABC/Wires