Iran on Friday summoned the envoy of Switzerland, which represents US interests in Tehran, to protest against the US seizure of Iranian oil from a Russian-operated ship near Greece, the foreign ministry said in a statement quoted by Iranian media.
The ministry called for the immediate release of the ship and its cargo, the IRNA state news agency quoted it as saying.
The United States on Wednesday imposed sanctions on what it described as a Russian-backed oil smuggling and money laundering network for Iran's Revolutionary Guards' Quds Force.
A spokesperson for the US Department of Justice declined to comment on the oil seizure.
Iran "expressed its deep concern over the US government's continued violation of international laws and international maritime conventions," IRNA and other media quoted the foreign ministry as saying.
A source at Greece's shipping ministry told Reuters on Thursday that the US Department of Justice had "informed Greece that the cargo on the vessel is Iranian oil".
It was unclear whether the cargo was impounded because it was Iranian oil or due to the sanctions on the tanker over its Russian links. Iran and Russia face separate US sanctions.
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.
The Iranian-flagged ship, the Pegas, was among five vessels designated by Washington on Feb. 22 - two days before Russia's invasion of Ukraine - for sanctions against Promsvyazbank, a bank viewed as critical to Russia's defense sector.
IRNA reported on Wednesday that its foreign ministry summoned the charge d'affaires of Greece's embassy in Tehran following the seizure of the cargo of a ship which was "under the banner of the Islamic Republic of Iran in Greek waters and he was informed of the strong objections" of Iran's government.
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".
