There is still a "major issue of trust" between the Greek government and its international creditors, the head of the eurozone's finance ministers said at the start of a "difficult" meeting to consider Athens' request for a fresh bailout.
The Greek parliament has backed government proposals for economic reforms in the hope of staving off bankruptcy and the possibility of crashing out of the euro.
But Dutch Finance Minister Jeroen Dijsselbloem, the Eurogroup president, said a deal was still a long way off.
Greek Prime Minister Alexis Tsipras has offered to implement tax rises and spending cuts in return for a financial lifeline of 53.5 billion euro (£38.5 billion) to prop up the country's economy.
During a late-night debate in Athens, Mr Tsipras acknowledged that the measures were a long way from the anti-austerity platform on which his radical Syriza party was elected.
Urging the Greek parliament to back his negotiating position, Mr Tsipras admitted he had made mistakes during his tenure and described the struggle with the creditors - the European Commission, European Central Bank (ECB) and International Monetary Fund (IMF) - as a battle.
"Now I have the feeling we've reached the demarcation line," he said. "From here on there is a minefield."
With Greece running out of money, and facing a July 20 demand for a three billion euro (£2.16 billion) payment to the European Central Bank, a series of meetings this weekend could hold the key to the country's economic future.
Mr Dijsselbloem said there was "still a lot of criticism" of the latest proposals from Athens.
"There is, of course, a major issue of trust: can the Greek government actually be trusted to do what they are promising to actually implement in the coming weeks, months and years?" he told reporters as he arrived at the Brussels meeting.
"Those are the key issues that will be addressed today."
The meeting of finance ministers from the single currency countries will be followed tomorrow by a summit of Europe's leaders from all 28 members, including British Prime Minister David Cameron.
Bank closures in Greece have been extended until Monday and Greeks are limited to withdrawing just 60 euro (£43) per day after the imposition of capital controls.
British tourists have been warned to take sufficient cash to cover expected costs and emergencies and to ensure they have supplies of their usual prescription medicines in case of shortages.