Failure to secure a deal could force a disorderly default which might in turn trigger a wider emergency across the euro zone as it struggles with the escalating sovereign debt crisis.
Asked if there was a risk of a disorderly Greek default, the senior troika official said: "Our objective is still to have a voluntary operation. If you ask me: is there a guarantee that there will be a voluntary operation? Of course there can never be a guarantee."
A deal with investors who hold 206 billion euros of Greek bonds is key to slashing the country's debt mountain, the euro zone's biggest at 160 percent of gross domestic product.
"The ongoing discussions are constructive, they are useful, but at this point it is too early to say what will be the result," the official, who declined to be named, told Reuters as the team of inspectors wrapped up a one-week visit to Athens.
He said the EU, IMF and European Central Bank "troika" aimed to clinch a deal with Greece on the details of the new bailout during its next inspection visit in mid-January.
The banks and funds in the steering committee of creditors, the IIF bank lobby and restructuring advisers are meeting in Paris on Friday after inconclusive talks in Athens this week.
Discussions have been tense, with stormy exchanges, a source involved in the talks said, adding that the IMF was playing
hardball on any guarantees the official sector will offer.
Greek officials have said repeatedly over the past days that they were optimistic a deal would be clinched swiftly after work on the deal -- albeit partially due to a general rethink on its scale -- has been dragging on for months.
"I believe we will make it because I have received positive signs from our consultations and discussions," Finance Minister Evangelos Venizelos told lawmakers on Friday.
The Greek government of former central banker Lucas Papademos wants to exchange existing bonds held by banks and other private sector players for a mix of cash and new bonds under an arrangement that would cut the nominal value of their holdings by 50 percent.
However there is disagreement over details including the credit status and interest coupons on the new bonds, and legal guarantees to be offered by the official sector which could significantly affect the real cost to investors.
Deutsche Bank Chief Executive Josef Ackermann, who also chairs the IIF, said on Thursday talks had run into the ground over demands from Athens that banks accept a haircut that would come to much more than the 50 percent losses originally agreed.
"Likelihood of a deal in 2011 is very low," a source close to the IIF said on Friday, adding that the Paris meeting was about technical issues rather than substantial decisions.
Charles Dallara, managing director of the IIF, said earlier this week that the deal could see banks rank on an equal footing with official euro zone lenders.
The senior troika official said this was indeed a new element in the discussions but that nothing was agreed on.
"There are different ideas being floated but so far no decisions have been made," the official said, adding that the conditions set in EU summits -- a voluntary deal with a 50 percent haircut to help slash Greece's debt to 120 percent of GDP by 2020 -- were tough because they required very high participation by private bondholders.
"What the discussions focus on right now is to see, given these parameters, can one come up with an offer that would be attractive to private bondholders? And then we will have to see whether there is sufficient response to the Greek authorities' offer."
Creditors are eager to find an agreement sooner rather than later and have lined up law firms Allen & Overy, White & Case and advisory firm Blackstone to negotiate, two sources familiar with the situation told Reuters earlier this week.
The plan was to reach a deal by the middle of next week, one source said, and if that didn't work it would come at the end of January at the earliest.
IMF and European officials are associated with the debt swap talks as observers at this stage, the troika official said, but the talks on both strands of the bailout deal -- the EU/IMF aid plan and the debt swap deal -- will come together early next year as the clock starts ticking on agreement.
Greece says it expects to receive about 90 billion euros in aid from the EU and the IMF early next year as part of the new bailout, with the cash shared between sweetening the debt swap deal, bank recapitalization and funds for Greece's budget.
But the troika official, who declined to confirm the amount of this aid tranche, warned Greece should not take it for granted and must first step up reforms.
"The EU member states and the IMF will not be willing to give Greece a huge amount of tens of billions of euros without being convinced that the money is well spent," he said.