Greek conservative leader Antonis Samaras has not yet signed a written commitment on implementing an austerity package, a source close to negotiations on a new bailout has said.
"So far Samaras has not given a letter of commitment and this is a problem," the source told Reuters on condition of anonymity.
His New Democracy party has declined to comment.
A separate source at the PASOK socialist party said its leader, George Papandreou, had already provided a signed undertaking to support the measures passed by parliament early yesterday.
The written pledge is one of the demands laid down by European leaders as part of negotiations on the country’s €230bn bailout.
Earlier, eurozone finance ministers put the second bailout on hold after Greece failed to meet conditions set by lenders in return for a rescue package it desperately needs to avoid default.
The eurozone had demanded that Greece's finance minister come to Brussels tomorrow with an extra €325m in budget cuts, as well as the written pledges from politicians.
However, Jean-Claude Juncker, head of the group of eurozone finance ministers, decided to switch the meeting to a conference call instead and prepare for previously scheduled talks in Brussels on Monday.
"I did not yet receive the required political assurances from the leaders of the Greek coalition parties on the implementation of the programme," Mr Juncker said in a statement.
He also said "further technical work" was needed between Greece and EU and IMF auditors "in a number of areas".
This included finding the extra €325m of savings and finishing an analysis of the sustainability of Greece's debt.
Greece desperately needs the €230bn rescue package - €130bn in fresh loans and a €100bn write-down on privately-held bonds - to avoid defaulting on €14.5bn in debt owed on 20 March.
The eurozone wants to ensure that the bailout deals will drastically reduce Greece's debt burden, from 160% of gross domestic product to 120% in 2020.
The EU, IMF and European Central Bank auditors have yet to provide ministries their report on debt sustainability owing to disagreements over how much the ECB should contribute to the write-down, said a source close to the negotiations.
