Greek Socialist leader Evangelos Venizelos has begun a third and final attempt at forming a new government, after Radical Left leader Alexis Tsipras and the conservative New Democracy party failed to do so this week.

Venizelos, whose PASOK party came in a distant third in Sunday's elections, is thought unlikely to succeed, which would force President Carolos Papoulias to call on parties to form an emergency coalition.

If that cannot be done by May 17, new elections will be called.

New Democracy won the most parliamentary seats in Sunday's election, followed by Tspiras' Radical Left and PASOK. But no party won enough seats to be able to put together a new government on its own.

Greek voters punished both PASOK and New Democracy for having pushed through tough economic austerity measures in return for huge international loans from Greece's European neighbors and the International Monetary Fund to avoid a default.

Tsipras had called on the two parties to renounce their support for the deep spending cuts but could not get enough support in parliament to hammer out a coalition.

EU leaders are pressuring Greece to carry through with the austerity plans.