The English-language television network, Press TV, reported that Greece's top refiner Hellenic Petroleum and Motor Oil Hellas were barred from purchasing Iranian crude after they defaulted on their orders.

A senior source at Hellenic Petroleum told Reuters on Tuesday that the refiner had suspended purchases of Iranian crude because sanctions imposed by the United States and the European Union made it impossible to meet its oil payments.

"We were using a Turkish bank all the time but we have to use an EU corresponding bank to make the payment from our Greek bank to the Turkish one and the EU banks are refusing that," the source said.

Motor Oil Hellas was not immediately available for comment.

The European Union will impose a full embargo on Iranian oil imports from July 1 as part of an international standoff over Tehran's nuclear program.

The U.S. and EU sanctions are designed to force Tehran to abandon its uranium enrichment activities which they suspect are part of a program to develop nuclear weapons. Iran has repeatedly denied this, saying its nuclear know-how is for peaceful purposes only.

Greece relied on Iran for more than half of its oil imports some months last year after traders and oil majors pulled the plug on supplies and banks refused to provide financing for fear that Athens would default on its debt.

Its financial difficulties mean Greece has struggled to find alternatives suppliers to Iran which supplied crude oil on favorable terms.

The United States has exempted Japan and 10 EU nations, including Greece, from financial sanctions because they have significantly cut their purchases.

But the exemption has little meaning because of the increasing difficulties in paying for oil shipments through international banks.

(Reporting by Marcus George; editing by Humeyra Pamuk and Jason Neely)

• A Permanent Economic Emergency