Iran’s Revolutionary Guard has seized two Greek oil tankers in helicopter-launched raids in the Persian Gulf about a week after the confiscation of Iranian oil from a tanker held off the Greek coast and its transfer to the US.

“The Revolutionary Guards Navy today (Friday) seized two Greek oil tankers for violations they have carried out in the waters of the Persian Gulf,” the IRGC said in a statement on Friday, shortly after Nour News -- a website affiliated to the secretary of Iran's Supreme National Security Council (SNSC), Ali Shamkhani -- warned of "punitive action" against Athens due to the seizure. The statement gave no further details about the alleged violations. 

The Greek tankers are Delta Poseidon and Prudent Warrior, and were captured near Asalouyeh off the coasts of Iran’s Bushehr Province and the Hendurabi island near Bandar Lengeh in Hormozgan province, respectively. 

Nour News said on Twitter, "Following the seizure of an Iranian tanker by the Greek government and the transfer of its oil to the Americans, Iran has decided to take punitive action against Greece."

The US Navy’s Mideast-based 5th Fleet said it was “looking into” reports about the seizure of the Greek oil tankers, with Commander Timothy Hawkins telling The Associated Press that the Navy was continuing to investigate, without further elaboration.

A US defense official, speaking on condition of anonymity, told the Washington Post that the two ships had apparently “come close to — but not in — Iranian territorial waters before the seizure,” adding that the ships also had turned off their tracking devices and neither had issued a mayday or a call for help.

In total, nine Greek nationals have been taken captive by the IRGC following the seizure of the two tankers.

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 EU sanctions. The United States later confiscated the Iranian oil cargo held onboard and chartered a tanker owned by Dynacom Tankers Management to send it to the United States. 

The Pegas was later released because of confusion about the sanctions over its owners. 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 as the two countries face separate US sanctions.

The aframax Lana, formerly named Pegas, was detained on April 15 by Greek authorities and was identified as the Russian-flagged Pegas and the assumption at the time was that it was laden with Russian crude.

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 defense sector.

Iran's Ports and Maritime Organization said last week that the tanker had sought refuge along the Greek coast after experiencing technical problems and poor weather. It called the seizure of its cargo "a clear example of piracy".

Earlier on Friday, the Iranian foreign ministry summoned a Swiss diplomat in Tehran to protest against the Pegas oil seizure. Switzerland represents US interests after relations were severed between Tehran and Washington in 1980.

"The Islamic Republic expressed its deep concern over the US government's continued violation of international laws and international maritime conventions," state media quoted the foreign ministry as saying.

On Wednesday, the United States imposed sanctions on what it described a Russian-backed oil smuggling and money laundering network for Iran's Revolutionary Guards' Quds Force.

In 2019, Iran seized a British tanker near the Strait of Hormuz for alleged maritime violations two weeks after British forces detained an Iranian tanker near Gibraltar, accusing it of shipping oil to Syria in violation of European Union sanctions.