Their letter says that "Unless business activity resumes soon, with a restoration of liquidity, there will be an explosion of unemployment that will be dramatic and will wipe out all the sacrifices made by the Greek people over the past five years."
More than one-in-four Greeks remain out of work, with the crisis and bailout austerity measures costing roughly a million jobs, mostly in the private sector.
Meanwhile, France's government spokesman says his country is trying to play mediator between Germany and Greece.
Stephane Le Foll said French President Francois Hollande's goal is "to find a sort of middle ground between the position of Germany and the position of Greece."
France, he said, is listening to those countries that accept the idea of a Greek exit from the euro but that it's important for his country "to facilitate, and above all keep a broad perspective" on the fact that the idea of European unity is also at stake.
France's Socialist government has been one of Greek Prime Minister Alexis Tsipras' few allies and is using France's role as a key, founding member of the European Union to try to gain a bit more flexibility.