Unless President Karolos Papoulias can broker a last-minute deal during the next few days, the country will be forced to conduct new elections next month, ekathimerini.com reported.
Greece still is struggling with a debt crisis that is threatening the eurozone.
Alexis Tsipras, leader of the Coalition of the Radical Left refused to agree to a enter a government that would include the Pasok, New Democracy and Democratic Left parties put forth by Pasok chief Evangelos Venizelos. He earlier reached agreement with New Democracy leader Antonis Samaras and Democratic Left head Fotis Kouvelis, who said he would agree only if the new administration included the Coalition of the Radical left as well as the New Democracy and Pasok.
Tsipras said he didn't have enough guarantees that the unity government would stop implementing austerity measures as part of the bailout Greece agreed to with the European Union and International Monetary Fund, ekathimerini.com.
"I want to stress that the rejection of this offer is not being given by me or by [Coalition of the Radical Left] but by the Greek people and their vote on Sunday," when parliamentary elections were held, Tsipras said.
His party wants to rewrite the loan agreement terms, despite insistence by the European Union and the IMF that only small changes can be made.
Tsipras also questioned whether the other parties willing to join the unity government also were committed to ditching measures such as canceling collective contracts, ending cuts to pensions and wages, going ahead with privatizations and public sector firings.
"They should give us clear answers to these key questions and forget public relations tricks," he said. "They should forget these tricks before poverty, recession, unemployment, emergency taxes, closed businesses and sackings finish off the country and its people."
Venizelos, who met with conservative Antonis Samaras, is the third leader to try to form a coalition since Sunday's elections failed to produce any clear-cut winners.
Also Friday, German Foreign Minister Guido Westerwelle said Greece's future in the eurozone "lies in the hands of Greece."
"We want to and we will help Greece, but Greece has to be ready to accept help," Westerwelle told German lawmakers. "If Greece strays from the agreed reform path, then the payment of further aid tranches won't be possible. Solidarity is not a one-way street."
Greece is to approve budget cuts worth $18.8 billion next month in return for financial help from the European Union and International Monetary Fund worth about $310 billion.