No Greek unity government can be formed, the head of a small moderate leftist party seen as the last hope for a coalition said Monday ahead of cabinet talks under President Carolos Papoulias.

"No unity government can emerge," Fotis Kouvelis, head of the Democratic Left party, told Antenna television, pointing to the refusal of the second-place radical leftist Syriza party to join a coalition.

"A government without Syriza would not have the necessary popular and parliamentary backing," said Kouvelis, a former Syriza senior member who broke away from the party in 2010.

Syriza leader Alexis Tsipras has already announced he will not attend the 1630 GMT talks under Papoulias, who has also invited the conservative New Democracy and socialist Pasok parties in a bid to break the impasse.

Kouvelis repeated his party's position that Greece must "immediately" start to disengage itself from an EU-IMF loan memorandum deemed to have plunged the country into a bottomless recession.

Party leaders have failed to break the deadlock for a week after May 6 election delivered an anti-austerity verdict and fragmented the political landscape, putting seven parties including a neo-Nazi group in parliament.

As it struggles to form a cabinet, Greece faces mounting threats of a loan freeze should it falter on promised structural reforms.

If a cabinet cannot be formed by Thursday, when parliament convenes, new elections will have to be called in June, a prospect many in the eurozone view with dread.

Opinion polls show that the radical leftist Syriza party, which soared to second place in the May 6 election on an anti-austerity ticket, rejecting Greece's EU-IMF bailout deal, would handily win a repeat ballot.

Faced with deep recession after two years of austerity, even the conservative and socialists who were previously in a cost-cutting coalition are now talking about renegotiating the terms of Greece's multi-billion euro (dollar) loan agreement to boost growth.

It is their only hope to secure the backing of Democratic Left, without whom they cannot form a viable government.