"With real GDP likely to decline another 4-5 percent next year after falling 6 percent this year, and further austerity testing social cohesion, risks to the EU-IMF program will remain substantial," the Institute of International Finance said.
"Doubts about debt sustainability will persist until growth resumes and government debt is reduced to a more palatable level," it said in a new global economic forecast.
"Continued deterioration in economic conditions restrain (Athens') ability to meet fiscal targets."
The IIF, which was deeply involved in arranging Greece's private sector debt writedown in March, raised the warning days after Athens completed a debt buyback that was crucial to the recast European Union-International Monetary Fund rescue program.
Greece is locked in its fifth year of recession, with its economy, under the pressure of severe government austerity policies, expected to shrink 6.5 percent in 2012.
According to the new deal reached with its official creditors, Athens was given more time to cut its debt load, now at 170 percent of GDP.
The country must reduce its debt to 124 percent of GDP by 2020 and 110 percent the following year.
But the IIF stressed that even that is not low enough.
"The debt sustainable level in Greece is not 120 percent, it's not 110, it's so much lower," said IIF chief economist Philip Suttle.
But Greece will have difficulty finding new lenders to back up the IMF and EU in sustaining it, he said.
"It seems to us that there is no private sector debt money that is going to be forthcoming to sit behind the official sector."
Meanwhile the IIF, which represents 470 major banks around the world, also expressed worry about political uncertainty in the wake of he early resignation of technocrat Prime Minister Mario Monti, who signaled plans to step down earlier this month.
"Italian political uncertainty has the potential to lead to a flaring up of Euro area tensions, including contagion to Spain," it said.
It said there were growing questions over whether the reforms launched by Monti would be sustained.
Former prime minister Silvio Berlusconi, for one, has said he will run for the job in the upcoming elections in part promising to roll back changes Monti introduced that have been praised elsewhere.