Two interesting quotes from an interview with Jens Weidman, President of the Bundesbank. Emphasis mine:
SPIEGEL: Are your alternatives truly realistic? As the example of Greece shows, a return to self-reliance and sanctions through interest rates is not an option. And stronger political integration requires a new contractual basis complete with lengthy parliamentary processes, and possibly even constitutional amendments and referendums.
Weidmann: I think both approaches are feasible in principle, which doesn't mean that they're easy. The second option does indeed require time-consuming changes to the European treaties and national constitutions. It's important to take these steps in the right order and not to introduce the common liability part now, which might be more convenient for some, just because political integration is expected to occur at some point in the future. But I also feel that it would be promising, contrary to many prophecies of doom, to pursue a path returning to the origins of the real framework. It has to be strengthened, however.
SPIEGEL: What exactly would happen if Greece were not to fulfill the conditions for the next tranche of aid payment? Would Greece no longer receive any money at all?
Weidmann: Part of the regulatory framework of the monetary union is to uphold agreements. This means that Greece must live up to its commitments, and that there will be no aid payments if these promises aren't kept. Otherwise we would be setting false incentives.
From another Spiegel article:
"'If something isn't done immediately then no option will be excluded,' the Süddeutsche Zeitung on Tuesday quoted an unnamed euro-zone source as saying. An anonymous European Commission source told the paper that 'absent a strong troika report, we will see a national bankruptcy at the end of the month.'"