Iran's supreme leader on Saturday said that the country's seizure of two Greek oil tankers late last month was a retaliation against Greece because it "stole" Iranian oil.

The Revolutionary Guards boarded and took control of the two vessels in the Gulf on May 27, citing "violations", which Greece condemned as "tantamount to acts of piracy".

The seizure came two days after Greece confirmed it would send 115,000 tonnes of Iranian oil from a Russian-flagged tanker, seized in April, to the United States at the request of the US Treasury and in line with the latter's sanctions regime.

"The Iranian fuel was stolen off Greece, then the brave (forces) of the Islamic republic made up for that by seizing an enemy ship," Ayatollah Ali Khamenei said in a televised speech marking the 1989 death of Ayatollah Ali Ruhollah Khomenei, who founded the Islamic republic.

"It's you who stole our oil," Khamenei said, addressing the country's "enemies."

Washington, Germany and France all called on Iran to release the vessels.

Athens said Iranian navy helicopters had landed gunmen on the two tankers to carry out the seizures.

Greek authorities had earlier seized the Russian-flagged ship, they said, to comply with sanctions introduced after Russia invaded Ukraine in February. The Iranian maritime authority had described that move as "international robbery".