The crews have remained on board the two tankers after Iran's Revolutionary Guards seized them on Friday, days after Athens confirmed it would deliver to Washington Iranian oil from a Russian tanker it seized.

Greece has condemned Tehran's detention of the two ships as "tantamount to acts of piracy" and warned its citizens not to travel to Iran.

Iranian foreign ministry spokesman Saeed Khatibzadeh said in a tweet that the crews were "safe and in good health".

He described ties between the two countries as having "always been based on mutual respect", adding that they "must not be hampered by deeply shortsighted miscalculations, including highway robbery on the command of a third party".

Iran's ports authority denied that the crews were arrested, according to a statement carried by the official news agency IRNA.

It added that they were being "protected... in accordance with international law. All the necessary services are being provided to them."

The Revolutionary Guards -- the ideological arm of Iran's military -- had said it seized the tankers "due to violations", without elaborating further.

Greece said one of the tankers had been sailing in international waters, while the second was near the Iranian coast when it was seized.

Nine Greeks are among the crews, the Greek foreign ministry said, without specifying the number of other sailors on board.

Athens said Iranian navy helicopters had landed gunmen on the two tankers.

Iran's action against the Greek-flagged tankers marks a sharp escalation in a diplomatic row that has raged since Greece seized a Russian-flagged oil tanker and its Iranian cargo last month.

Athens has linked the tanker seizure in Greece to sanctions imposed on Russia following its invasion of Ukraine in February.

Iran's foreign ministry demanded Friday that Greece release the vessel, saying the planned transfer of its cargo to the US was a "clear violation" of international law.

The United States reimposed crippling sanctions on Iran in 2018, after then president Donald Trump pulled out of a nuclear agreement between Tehran and major powers. Its once-lucrative oil exports are a major target.