German finance minister Wolfgang Schaeuble says Greece needs more financial help.
Mr Schaeuble says the current aid package for Greece is insufficient, and there is a real risk of default if funds are not released soon.
In a letter to the European Central Bank and International Monetary Fund leaders, Mr Schaeuble says a new package is needed, the BBC reports.
He suggests investors should also be paying for the bailout, as well as taxpayers.
A 110bn euro bail-out package for Greece was agreed last year.
Greece has implemented a series of austerity measures designed to cut its budget deficit - measures that have proved deeply unpopular, provoking widespread protests as the cuts have begun to bite.
On Wednesday, official figures showed that the country's unemployment rate rose to 16.2% in March, up from 15.9% in February.