Greece seized the Russian Pegas oil tanker, which had been renamed Lana in an attempt to conceal the ship’s identity, along with 19 crew members in late April.
Athens News Network reported the ship was planning to transfer the fuel to another tanker at sea.
The vessel was seized in compliance with European Union sanctions on Russian oil exports, which prevent Russian tankers from docking in European ports, although the ship was found to be carrying 115,000 tonnes of Iranian oil.
In pictures: seized Iranian oil tankers
Greece is increasingly complying with US pressure to curtail Russia's oil exports. Before the Ukraine war, Greek tankers carried a large percentage of Russian oil to market but the EU plans to phase out Russian oil imports, except in special circumstances.
Some countries, including the Czech Republic and Slovakia, have appealed for more time to phase out Russian oil and find alternative supplies, or opposed the oil blockade altogether, such as Hungary.
Greek tankers will still be allowed to carry Russian oil, but Athens is complying with Brussels' wish to block Russian tankers from EU ports.
"Following a request from the US justice system, the oil is to be transferred to the United States at the expense of that country," a spokeswoman for the Greek port police told AFP on Wednesday.
Oil tankers subjected to trade sanctions often undertake measures to conceal their ownership.
Apart from changing names, some ships carrying illegal oil shipments switch off devices known as transponders, that tell other ships of their location, stopping what is known as the Automatic Identification Signal and conducting ship to ship transfers of oil at sea.
Sometimes AIS signal devices can be modified to give false data, despite the efforts of transponder manufacturers to prevent this kind of tampering.
Tehran strongly protested the Greek decision, calling it "international robbery," the Iranian maritime authority said.
"The Islamic Republic of Iran will not waive its legal rights and expects the Greek government to adhere to its international obligations in the field of seafaring and shipping", the Ports and Maritime Organisation of Iran said.
Iran's foreign affairs ministry late Tuesday called on the Greek government, via the International Maritime Organisation, to release the tanker and its crew, adding that "Americans unloaded the cargo of the ship."
Athens did not respond immediately to the Iranian protests and provided no further details about the oil or how it would be transferred to the United States.