The International Monetary Fund disagrees with EU projections on Greece's debt sustainability and wants to wait until a clearer outlook emerges before signing off on the next tranche of financial support to Athens, EU officials said on Thursday.

The IMF, which together with the European Commission and European Central Bank comprises the 'troika' of inspectors in Greece, believes the EU's debt projections are too optimistic and wants to wait until after a euro zone summit on Sunday to see if discussions there produce a clearer picture on how sustainable the debt levels are.

"The IMF thinks that estimates by other parts of the troika are over-optimistic," one source said, while another added: "The IMF will definitely want to see what the Eurogroup and the European Council come up with first," referring to this weekend's meeting.

The troika was expected to publish its latest report on Greece on October 24, but there was pressure to complete it before the October 23 summit. The report is expected to indicate whether Greece is doing enough to on its budget to justify receiving the next, 8 billion euro installment of aid.

If Sunday's summit agrees on a deeper involvement by the private sector in helping to reduce Greece's debt burden, that may make the debt stock more sustainable and allow the IMF to sign off on the release of the next tranche, possibly in November, the sources said.