But, in an indication that Tehran's "counter-sanctions" were of little impact, Spain's biggest refiner said it had already replaced Iranian crude with Saudi Arabian oil months ago.
Iran has played a tit-for-tat game over crude shipments since the European Union decided in January to stop all Iranian oil imports as of July, part of a range of tough new sanctions aimed at forcing Tehran to curb the atomic work that the West suspects is part of a nuclear weapons programme.
Talks between Tehran and six world powers aimed at easing the nuclear stand-off are set to resume in Istanbul on Saturday, and could pave the way for an easing of sanctions and might lift the threat of Israeli air strikes on Iran.
EU states have sought alternative oil supplies ahead of July's deadline, with Iran threatening to cut exports first, something Iran's Press TV said was well under way.
"Tehran has cut oil supply to Spain after stopping crude export to Greece as part of its counter-sanctions," Press TV said, citing unidentified sources, adding that a similar move was being considered for Germany and Italy.
A spokesman for Spanish refiner Repsol said it had not been buying Iranian oil for months.
"No crude out of Iran for us since January," he said.
The EU was the second biggest buyer of Iranian oil after China before the embargo, which is a direct strike on the OPEC member's biggest source of export income.