The International Monetary Fund said an agreement remained far-off after a five-month stalemate with Greece's anti-austerity government, which faces being unable to pay huge debts at the end of the month.
Eleventh-hour talks in Brussels between Greek Prime Minister Alexis Tsipras and European Commission chief Jean-Claude Juncker meanwhile broke up without reaching a deal on reforms in exchange for bailout cash.
"There are still major differences between us in most key areas," IMF spokesman Gerry Rice told reporters in Washington. "There has been no progress in narrowing these differences recently. Thus we are well away from an agreement."
It said the key disagreements were on pensions, taxes and financing.
Also Read
The IMF is the most hardline of Greece's three bailout monitors -- the others being the European Commission and European Central Bank -- who have demanded tough reforms in exchange for unlocking the remaining 7.2 billion euros ($8.1 billion) of its 240-billion-euro rescue package.
Without fresh external funding when the bailout expires on June 30, cash-strapped Greece is set to default on its debts, meaning it could crash out of the eurozone despite benefitting from two international bailouts since 2010.
The crisis is now set to go to the wire with a Eurogroup meeting of eurozone finance ministers in Luxembourg on June 18 seen as the last chance to seal a deal in time to get it through national parliaments by the end of the month.
EU president Donald Tusk issued an unprecedented warning to Tsipras, telling the Greek government to stop "gambling" and saying the Eurogroup meeting would be "really crucial" and "decisive"
"There is no more time for gambling. The day is coming, I am afraid, that someone says the game is over," he told a press conference.