Speaking in the Bundestag lower house of parliament, German Finance Minister Wolfgang Schaeuble said: "Even Greece has made substantial progress in reducing its budget deficits."

"But it is clear that all obligations agreed in its programme at the start of the year must be stuck to," he insisted.

"If this doesn't happen, then confidence would be destroyed again and there would be a heightened danger of contagion throughout the eurozone. Therefore there can be no new negotiations," Schaeuble said.

He pointed to a report due out next month by international auditors assessing Greece's progress towards making some 11.5 billion euros ($14.7 billion) in cuts.

This will determine whether Greece receives a slice of 31.5 billion euros it needs to stay afloat.

ECB president Mario Draghi later met Greek Prime Minister Antonis Samaras at the central bank's headquarters in Frankfurt to discuss mainly Greece's progress on finalising those cuts.

"Both parties agreed that Greece has already taken significant steps towards budgetary consolidation and economic modernisation but that major challenges remain," a ECB statement said.

Samaras assured ECB officials of Greece's commitment to getting the country's reforms "back on track" to return Greece to competitiveness and its public finances and economy to a solid footing, it added.

Schaeuble, in his wide-ranging speech, had also stressed the independence of the ECB and called for it to be respected.

The ECB's decision last week to buy unlimited volumes of the bonds of struggling eurozone countries in a bid to drive down their borrowing costs has come under fire in Germany amid concerns the bank is overstepping its mandate.

But Schaeuble emphasised that "nothing could happen" in the individual member states without commitment to deep structural reforms of their economies.

"That is what we call conditionality. And this conditionality is a fundamental precondition for aid programmes," the minister said.