Germany's economy minister said in an interview Tuesday that possible talks on reducing Greeces debt were not imaginable before Athens implements reforms.
Sigmar Gabriel told Stern's online edition that simply cancelling debt without many fundamental changes in Greece would not reap any benefit.
He was quoted as saying that one could "only talk about the possibility of decreasing the debts when the Greek government also shows that it is implementing reforms".
Otherwise, he warned, the debts would immediately mount up again the day after any debt restructuring.
Germany is currently standing firm in refusing to negotiate a restructuring of Greeces debt.
Regarding requests by Athens on the issue, German finance ministry spokesman Martin Jaeger said Monday: "I can see no reason to enter into discussions."
EU leaders have warned Greek Prime Minister Alexis Tsipras to present new bailout proposals at an emergency summit later Tuesday or risk leaving the single currency, after Greeks rejected further austerity in a referendum Sunday.
Gabriel, a Social Democrat who is also Chancellor Angela Merkel's deputy in a left-right coalition, also told Stern that from today's viewpoint, it had been "very naive" to allow Greece to join the eurozone.
"Worse, however, is that everyone watched on for much too long" as the country sunk deeper into crisis, he said.
Gabriel warned a lesson learned was "never again to look away when a country doesn't keep to the rules in Europe", saying this also applied to non-financial matters.