Earlier, Iran's semi-official Fars news agency said Tehran had refused shipment of 500,000 destined for Greek refiner Hellenic Petroleum, but a Hellenic official denied it. "That has nothing to do with us ... all supplies from Iran have been processed normally," the official told Reuters.
The European Union decided in January to stop importing Iranian crude as of July 1, the latest in a series of international sanctions aimed at forcing Tehran to halt its sensitive nuclear work, as demanded by the UN Security Council.
Iran stopped selling crude to British and French companies last week after the oil minister said Tehran would cut oil exports to "some" European countries.
Brent crude surged to over $125 a barrel on Friday amid fears of an escalation of tensions after the UN nuclear agency issued a report showing Tehran had increased its most sensitive nuclear work which the West says is part of a weapons programme, a charge Tehran denies.
The price of oil has increased over the fear of tightening supplies, including a threat from Tehran to close a vital oil shipping route, the Strait of Hormuz, if attacked.
Israel and the United States have not ruled out military action if diplomacy fails to stop Iran getting nuclear weapons. Iran says its nuclear programme is for peaceful purposes and has promised a "painful" response to any attack.
The world's top oil exporter Saudi Arabia said it would compensate for any oil shortage in the market and the European Commission has said that the bloc would not be short of oil if Iran stopped crude exports, as it has enough in stock to meet its needs for around 120 days.
While Europe would suffer more directly from the lack of Iranian crude than the United States, which does not buy oil from Tehran, the knock on effect of a disruption would drive up prices across the globe.
Greece is considered more vulnerable than most as it is suffering a major economic crisis and has been reliant on Iran for almost one quarter of its oil imports. Traders told Reuters on February 24 that Swiss-based Totsa, the trading arm of French oil major Total, and trading house Mercuria were in separate negotiations with Hellenic to help it replace Iranian crude.
Glencore, a leading Swiss-based commodities trader and one of the few that conducted business with Greece during the debt crisis, may also boost supplies, trading sources have said. Hellenic would pay back the traders with refined products, which could then be sold in Greece or abroad.