Iran called Friday for the release of a Russian-flagged tanker seized by Greece, saying the transfer of its Iranian cargo to the US was a "clear violation" of international law.

Athens confirmed on Wednesday it will send 115,000 tonnes of Iranian oil from the tanker Pegas to the United States at the request of the US Treasury which overseas the enforcement of sanctions on Iran.

Iran's foreign ministry summoned the Swiss charge d'affaires, whose country has handled US interests in Iran ever since ties were cut between Tehran and Washington following the 1979 Islamic revolution.

The ministry protested "the US government's pressure and intervention" that led to the seizure of the ship by Greek authorities last month.

EU member Greece said at the time it was enforcing sanctions the bloc had imposed on Russia following its invasion of Ukraine in February.

Iran accused the United States of a "clear violation of the law of the sea and the relevant international conventions" and "called for the immediate lifting of the seizure of the ship and its cargo."

A source in the Greek coastguard told AFP the transfer of the cargo to a Liberian-flagged vessel for onward delivery to the United States had began on Thursday.

The operation is going to "take some days", the source added.

The Iranian foreign ministry already summoned the Greek charge d'affaires to protest the seizure on Wednesday.