An International Monetary Fund official said the Fund's participation in the second Greek bailout is essential to make it work.
But he also told journalists that the complex 237 billion euro ($314 billion) deal agreed Tuesday remains at immediate risk to the need to get a large commitment from private sector holders of Greek bonds to participate in a debt writedown.
"The deal has been... negotiated with private creditors' representatives. So all the cards have been stacked up for it to succeed," said the official, who declined to be identified.
"But of course there's huge implementation risks over the next few days. And we have to be realistic, it's not yet in the bank."
The Fund itself also now has to decide its level of participation in the $110 billion euros of official aid being offered in the new rescue package, the official said.
IMF chief Christine Lagarde is preparing to propose a new financing deal for Greece to the IMF board.
But she will face concerns among some members that the fund has already pumped a record 20 billion euros into the country in the first bailout, without succeeding in stabilizing the country's finances.
The official said the IMF's participation is crucial to make sure both the private sector and EU public sector also join in.
The IMF plays "a catalytic role" in such deals, he said.
"The private sector has constantly indicated in these discussions with Greece that they regard it as an essential aspect to give credibility to the financing package, and their contribution to Greece, that the Fund be present as a co-financier."
"If we are there, both the private sector and the official sector will also come along to support a country. If we are not there, most likely that support will not be forthcoming."
The official declined to confirm reports that the IMF plans to contribute 13 billion euros, on top of 10 billion euros still not disbursed from the original loan.
"All in all (it will be) a very very sizable financial commitment that, again, is important to unlock the private sector involvement and the official sector support from the Europeans."
He said the plan for banks to write off 53.5 percent of the 206 billion euros in Greek debt they hold still hinges on nearly all of the debt holders signing on to the deal.
Banks and other investors, like hedge funds, have to decide whether to agree to take partial losses on their debt, or stay out and "run the risk of letting the whole deal fall through and potentially face a full outright default," he said.
"So if those are the considerations, those investors might be better off in coming in."