"The Revolutionary Guards Navy today seized two Greek tankers for violations in Gulf waters," said a Guards statement, quoted by Iranian state news agency IRNA. It gave no further details and did not say what the alleged violations were.
Greece's foreign ministry said an Iranian navy helicopter landed on Greek flagged vessel 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.
It said a similar incident took place on another Greek-flagged vessel near Iran, without naming the ship, adding both actions violated international law and Greece had informed its allies, as well as complained to Iran's ambassador in Athens.
Greece-based Delta Tankers, which operates the Delta Poseidon, could not be immediately reached for comment.
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 United States later confiscated the Iranian oil cargo held onboard and plans to send it to the United States on another vessel, Reuters reported on Thursday.
The Pegas was later released, but the seizure inflamed tensions at a delicate time, with Iran and world powers seeking to revive a nuclear deal that Washington abandoned under former President Donald Trump.
Earlier on Friday, Nour News, which is affiliated to an Iranian state security body, 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."
It did not say what kind of action Iran would take.
The Pegas was among five vessels designated by Washington on Feb. 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 semi-official Tasnim news agency later quoted an unnamed source as saying: "There are 17 other Greek vessels in the Persian Gulf, which could be seized by the Revolutionary Guards if Greece continues its mischievousness."
"Informed sources also stress that Greece should take compensatory measures towards the Iranian oil tanker as soon as possible," said Tasnim.
A maritime security source said the other tanker seized on Friday was the Greek-flagged Prudent Warrior. Its operator, Greece-based shipping firm Polembros, told Reuters there had been "an incident" with one of its ships, without elaborating, adding it was "making every effort to resolve the issue."
US advocacy group United Against Nuclear Iran (UANI), which monitors Iran-related tanker traffic through ship and satellite tracking, said Prudent Warrior was carrying a cargo of Qatari and Iraqi oil, while the Delta Poseidon was loaded with Iraqi oil.
Each vessel was carrying approximately one million barrels, it said.
"This should have direct implications on the JCPOA (Iran nuclear) negotiations and further stalling any chances of reviving a deal," Claire Jungman, chief of staff at UANI, told Reuters.
A spokesperson with the US Fifth Fleet in Bahrain said it was aware of the reported seizures and was looking into them.
Also on Friday, Iran summoned an envoy of Switzerland, which represents US interests in Tehran, to protest against the Pegas oil seizure, the Iranian foreign ministry said.
"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.
A spokesperson for the US Department of Justice declined to comment on the oil seizure.
IRNA quoted Iran's Ports and Maritime Organization as saying 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".
The United States on Wednesday imposed sanctions on what it described as a Russian-backed oil smuggling and money laundering network for the Revolutionary Guards' Quds Force.
In 2019, Iran seized a British tanker near the Strait of Hormuz for alleged marine 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. Both vessels were later released.