EU leaders have warned Greeks that rejecting creditors' proposals in a snap referendum called for Sunday would mean leaving the euro.
German Vice Chancellor Sigmar Gabriel said the vote would be "yes or no to the eurozone".
Greek Prime Minister Alexis Tsipras has urged a no vote but insists he wants Greece to stay in the euro.
Talks between Greece and its creditors broke down last week, leading to Greek banks having to shut this week.
Global stock markets fell sharply on Monday.
As well as Mr Gabriel, the leaders of the eurozone's other two largest economies said Greek voters would effectively be deciding next Sunday whether or not they wanted to stay in the eurozone.