"It would be helpful if clarity is achieved as soon as possible on which path Greece wants to take," Finance Minister Wolfgang Schaeuble told the Hamburger Abendblatt newspaper.
After EU leaders agreed last week on measures to help Greece and stabilize the 17-member currency union, Greek Prime Minister George Papandreou stunned his euro zone partners and shook financial markets by calling a referendum on the planned 130 billion euro bailout package.
Chancellor Angela Merkel and French President Nicolas Sarkozy have summoned Papandreou to crisis talks in Cannes on Wednesday before a summit of G20 world economies.
German European Commissioner Guenther Oettinger said Papandreou's actions had "made the situation considerably worse for countries which don't have the highest credit rating, and the danger of further setbacks is rising."
He told Die Welt newspaper that the Greek leader should have given European leaders advance warning of his referendum plans at their twin summits last week. The mass-circulation Bild daily said "even the chancellor was taken by surprise."
"My worry is that we will have an unstable situation until the referendum. The Greeks have many good grounds to vote for the package. The steps we decided on are a good opportunity for their country. But there's a lot of frustration and resignation in Greece. If the Greeks actually vote no, the consequences are unforeseeable," said the European energy commissioner.
Schaeuble said in an interview in the Hamburger Abendblatt that he "assumed Greece is aware of its responsibility and will go along with the measures that were agreed together and unanimously." He echoed that comment in a separate interview with the Financial Times Deutschland.
But Manfred Weber, the German vice-chairman of the center-right bloc in the European Parliament, warned that Athens was "playing with fire. If the Greeks vote no, there will be no alternative but a sovereign default."