Antonis Samaras, leader of New Democracy, one of three parties in a new national unity coalition struggling to avert a disastrous default, says there is no need to provide a written guarantee because his word can be trusted.
But international lenders, weary of Greece's failure to deliver on fiscal targets during two years of financial crisis, insist on the written statement, fearing its politicians may otherwise try to wriggle out of their commitments, especially with an election likely in February.
Representatives of the European Commission, the European Central Bank and the International Monetary Fund -- the "troika" which monitors Greek compliance with its rescue deals -- discussed the issue with Samaras in Athens on Saturday.
"Regarding the discussion we had with the troika and specifically on the written reassurances, I repeated to them my position on the issue," Samaras told reporters after the talks.
Samaras, a Harvard-educated economist known for his fiery rhetoric, said New Democracy had proven its commitment to the terms of the bailout by backing the coalition led by technocrat Prime Minister Lucas Papademos and its new budget.
Papademos's coalition, which comprises New Democracy, the Socialists of fallen premier George Papandreou and the small far-right LAOS party, won a parliamentary vote of confidence on Wednesday and submitted a draft budget to parliament on Friday.
Samaras's stance potentially puts at risk the next aid tranche for Greece worth 8 billion euros (about $11 billion) that it needs by mid-December to meet debt repayments. Failure would mean default.
The new bailout negotiated last month envisages a further 130 billion euros meant to keep the Mediterranean country of 11 million people financed until 2014.
Political analysts said they expected some face-saving compromise would be found to end the standoff.
"One way or another, there will be a way to bypass this problem. I believe there will be a solution that will not force Samaras to give a written commitment, and it will come soon," Costas Panadopoulos, head of pollster ALCO, told Reuters.