Tehran summoned the envoy of Switzerland on Friday to protest against the US seizure of Iranian oil from a ship near Greece that is believed to be operated by Russia.
Switzerland represents US interests in Tehran.
“The Islamic republic expressed its deep concern over the US government's continued violation of international laws and international maritime conventions,” Iran's foreign ministry said, according to a report by the semi-official ISNA news agency.
The ministry called for the immediate release of the ship and its cargo.
The Iranian-flagged Pegas was among five vessels designated by Washington on February 22 — two days before Russia's invasion of Ukraine — as part of sanctions against Promsvyazbank, a lender that is considered ritical to Russia's defence sector.
Promsvyazbank is believed to have controlled the vessel.
Greece seized the tanker, which was formerly Russian-flagged but was renamed Lana on March 1 to conceal its identity, along with 19 crew members in late April.
Athens News Network reported that the ship was planning to transfer its cargo to another tanker at sea.
The vessel was seized in compliance with EU sanctions that prevent Russian oil tankers from docking at European ports. It was found to be carrying 115,000 tonnes of Iranian oil.
Confiscated oil sent to US
The confiscated oil was being sent to the US aboard another vessel, three sources told Reuters on Thursday.
It was unclear whether the cargo was impounded because it was Iranian oil or due to the sanctions on the tanker due to its being under Russian control.
Iran and Russia are facing separate US sanctions.
The vessel's Russian owner, Transmorflot, was subsequently designated on May 8. The tanker, which has flown the Iranian flag since May 1, has remained near Greek waters since then.
A source at Greece's shipping ministry said on Thursday that the US Department of Justice had “informed Greece that the cargo on the vessel is Iranian oil”.
“The cargo has been transferred to another ship that was hired by the US,” the source said, without providing further details.
The US on Wednesday imposed sanctions on what it described as a Russian-backed oil smuggling and money laundering network for Iran's Islamic Revolutionary Guard Corps' Quds Force, even as Washington tries to revive a nuclear deal with Iran.