The head of the euro zone's permanent bailout fund, the European Stability Mechanism (ESM), has expressed concern that not all of the bloc's struggling states would be able to complete their reforms, a newspaper report says.

"My biggest worry is that some crisis-stricken countries do not have the political power to persevere with their painful but effective reform programs until the end," Klaus Regling was quoted as saying in an advance copy of an article due to appear in Germany's Rheinische Post newspaper on Saturday.

"That would be a catastrophe," he added.

Regling warned against a Greek exit from the single currency bloc, saying this would be the most costly solution.

He also said the euro zone's crisis strategy was working better than many people acknowledged and that the region was more than halfway through national structural adjustments.

(Reporting by Michelle Martin; Editing by Michel Roddy)