Talks are progressing on private bondholders' contribution to Greece's international bailout but there is no guarantee they will lead to a voluntary deal involving the bulk of its creditors, a senior troika official said on Friday as EU, IMF and ECB inspectors wrapped up a one-week visit to Athens.

Greece is scrambling to put together a new bailout plan with the International Monetary Fund, the European Union and banks that involves a complex debt-cutting deal before major bond redemptions come due in the first quarter of next year.

The official said the troika aimed to clinch a deal with Greece on the details of the new bailout during its next inspection visit in mid-January; that was roughly in line with a target the Greek government has set publicly.

Key questions in recent weeks have been the shape of a final deal and the actual cost to be imposed on creditors, as well as how many would finally sign up to the private sector debt swap.

The ongoing (debt swap) discussions are constructive, they are useful, but at this point it is too early to say what will be the result, said the senior troika official who declined to be named.

A deal with investors who hold some 206 billion euros of Greek bonds is key to slashing the country's debt mountain, which at around 160 percent of gross domestic product is the heaviest in the euro zone.

The 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.

But agreement has been held up by wrangling over issues including the credit status and interest coupons on the new bonds to legal guarantees to be offered by the official sector.

Failure to secure agreement could force a disorderly default which might in turn trigger a wider emergency across the euro zone as it struggles to cope with the escalating sovereign debt crisis.

Asked if there was a risk of a disorderly Greek default, the official told Reuters: 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.

Charles Dallara, managing director of the Institute of International Finance bank lobby that spearheads the debt swap talks for banks 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.

Greece says it expects to receive about 90 billion euros in aid early next year as part of the new bailout, with the cash shared between sweetening the debt swap deal, bank recapitalization and funds to cover Greece's financing needs.

But the senior official, who declined to confirm the amount of this aid tranche, warned that 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. It is very important that the Greek authorities understand that, the official said.

(Reporting by Ingrid Melander; editing by James Mackenzie/Patrick Graham)

International Monetary Fund