Speaking on a documentary about the eurozone crisis due to air on public broadcaster ZDF next Tuesday, German finance minister Wolfgang Schäuble said: "We want Greece to stay in the eurozone. But Greece has a lot of things to do. That has not yet been decided."
"There are doubts over whether Greece has fulfilled its commitments up until now. These doubts must be dispelled in the future," added Schäuble, according to a statement released in advance byZDF.
Eurozone officials are poring over a Greek request for the terms of its bailout to be extended by two years to 2016, allowing it to spread out the pain of the austerity measures it has signed up to in return.
Such an extension, however, could add up to €30 billion to an already huge bailout bill.
A spokesman for the head of the eurozone finance ministers' group, Jean-Claude Juncker, said earlier on Friday that they would hold a conference call on Wednesday.
A team of auditors from the International Monetary Fund, the European Union and the European Central Bank is putting the finishing touches on a report that will decide whether Athens gets the next tranche of bailout cash
to stay afloat.
Greek Prime Minister Antonis Samaras has been trying to persuade his coalition allies to accept new austerity measures to close that deal but they have baulked at labour reforms demanded by the troika of creditors.
Without the aid payment, Samaras has warned, the country could be broke by the middle of next month.
Schäuble also lashed out at some Greek politicians who have sought to deflect the blame away from Athens.
"If politicians in Greece try to convince people that someone in Brussels, Europe or Germany is responsible for their problems, then they are irresponsible demagogues," he said, according to the text.
"Blame for the situation in Greece is down to nobody other than Greece and the Greek politicians," he said.