The Iranian Foreign Ministry called for the immediate release of the ship and its cargo, IRNA reported.
"The Islamic Republic expressed its deep concern over the US government's continued violation of international laws and international maritime conventions," the ministry said in a statement.
On Wednesday, Iran summoned the Greek charge d’affaires over the matter.
A day later, a Greek official said that following a “judicial intervention by US authorities concerning the ship’s cargo” the ship’s oil was being transferred to another vessel off the port of Karystos on the Aegean Sea island of Evia, AP reported.
Three sources familiar with the matter told Reuters on Thursday that the US plans to send the cargo to the United States aboard another vessel.
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, adding that the seizure of its cargo was "a clear example of piracy".