Sri Lanka and a group of its creditor nations, including Japan, France and India, on Wednesday reached an agreement in principle on a debt rework of $5.9 billion of outstanding public debt. That followed a deal between the country and the Export-Import Bank of China in October on about $4.2 billion of loans.

But the bondholder group, which represents creditors holding some of the country's $12 billion of outstanding bonds, said a lack of transparency between public and private creditors was making it more difficult for them to strike a deal with Sri Lanka that is compliant with IMF rules and that provides "fair and equitable" debt treatment.

"The Group finds it regrettable that there remains such a significant lack of transparency on the part of official sector creditors despite the Group's efforts so far to act as a constructive counterparty," the Ad Hoc Group of Bondholders said in an emailed statement.

The visibility of terms and conditions attached to deals struck between indebted countries and their official and private creditors have recently moved centre stage.

Last week, objections from official creditors derailed an agreement in principle between Zambia and its bondholders to restructure the African nation's international debt. The group of Zambia's bilateral creditors, including France, China and India, said the terms of that proposed deal were not comparable to the relief official creditors offered.

The Sri Lanka Ad Hoc Group of bondholders said it remained committed to reaching an agreement with the Sri Lankan authorities as quickly as possible to find a sustainable solution to the country's international bond debt challenges.

The Ad Hoc Group is advised by Rothschild & Co on the financial side and by White & Case LLP on legal matters.

(Reporting by Karin Strohecker, editing by Libby George, William Maclean)