(Reuters) - The creditor institutions which have agreed a draft third bailout for Greece have "serious concerns" about the sustainability of Greek public debt although they believe a nominal haircut can be avoided, a debt sustainability analysis seen by Reuters on Thursday showed. "The high debt to GDP and the gross financing needs resulting from this analysis point to serious concerns regarding the sustainability of Greece's public debt," said the analysis, adding far-reaching reforms were needed to address the worries. "In sum, an appropriate combination of extension of maturities and grace periods for principals and interests would allow to bring Greece debt back to a sustainable level in gross financing needs terms without the need for a nominal haircut as stated by the Euro Summit of 12 July," it added.