The authorities seized the ship in accordance with EU sanctions introduced after Russia invaded Ukraine in February.
According to information at the time, the tanker was carrying 115,000 tons of Iranian oil.
“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.
Tehran strongly protested the 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 added, in a statement posted on its website.
Iran’s foreign affairs ministry late Tuesday called on the Greek government, via the International Maritime Organization, 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.