Greece was pulled back from impending default Saturday, when eurozone finance ministers signed off on a vital loan installment. But the country's international creditors are showing more concern over whether it can service its debt in the long run.

Athens will get a €12 billion ($17.39 billion) tranche of its existing €110 billion rescue package by 15 July, in time to meet several bond repayment deadlines this month and next. The eurozone and the International Monetary Fund will also continue to prop up Greece's struggling economy in the coming years, with a second package of aid loans to be finalised by September.

While the renewed commitments save Greece from immediate collapse, even its international creditors — long the biggest optimists on the country's prospects — are warning that getting down a debt of 160% of economic output will be a difficult balancing act.

In an illustration showing several scenarios for Greece’s debt load, growth of just one percentage point below expectations would leave Greece's debt at around 170% of gross domestic product past 2020, with the graph pointing firmly upward.

The report, the basis for the ministers' decision to release the July aid installment and prepare a new bailout, is the most pessimistic assessment from the commission yet. Private analysts and economists have long questioned the sustainability of Greece’s debt. However, the European Union, the European Central Bank and the IMF have so far, at least publicly, upheld their belief that Greece's situation is manageable.

The Commission still maintains that it is “not unrealistic to assume” that Greece can cut its deficits to the targets set out in its bailout programme, and thereby slowly chip away at its debt. But the report puts a sizable question mark over the country's ability — and willingness — to implement the reforms its creditors say are necessary to get the economy growing again.

The warning has a clear ring to it, following weeks of sometimes dramatic back and forth between Greek authorities and the country’s international creditors, which culminated earlier this week in the narrow passage of unpopular new austerity measures through parliament amid violent demonstrations in Athens.

For the first time, the Commission’s report also contains a section on debt restructuring — including a scenario for a 40% haircut, a forced reduction in the value of Greek bonds.

The EU has so far ruled out any haircuts on bonds, and in its report the Commission maintains that the negative consequences of a restructuring would outweigh any gains from debt restructuring.

A 40% haircut would devastate Greek banks, wiping out the capital cushions and triggering massive deposit flight, the Commission warns. Restructuring Greece’s debt also risks “creating a permanent shift in investor sentiment and leads to self-fulfilling prophecies for other vulnerable member states,” — shorthand for already bailed out Ireland and Portugal, as well as weak states like Spain or Italy.

The report highlights that Greece’s destiny will likely be decided by what happens within the country as well as by outside conditions it has little influence over, such as global economic growth that would provide it with a better market for exports.

Those factors are likely to overshadow any decisions on a second bailout package, which merely buys Greece more time, and the exact nature of private-sector involvement, the main open issue in the debates on a new bailout.