German Finance Minister Wolfgang Schäuble has offered to exchange financially-crippled Greece for debt-ridden Puerto Rico in the single-currency eurozone.

Schäuble, a harsh critic of the leftist Athens government, quipped: "I offered my friend (US Treasury secretary) Jack Lew these days that we could take Puerto Rico into the eurozone if the US were willing to take Greece into the dollar union."

In a public address in Frankfurt, the hawkish German minister went on: "He thought that was a joke."

Given the current financial situation of Greece, this may well be the tone of the backdoor negotiations. Nevertheless, the 72-year-old German minister's decision to publicly take a dig at the country has raised eyebrows.

Puerto Rico, a US territory, owes its creditors $73bn in debts which its governor Alejandro García Padilla has refused to pay. The financial situation of Greece and Puerto Rico are strikingly different and not even remotely comparable.

Schäuble's remarks have come at a time when Greece has submitted the economic reform proposals, currently being vetted by European authorities, even as the creditors brace for last-ditch efforts to engage with Greece.

Schäuble's comments also highlight the growing impatience in some political quarters in Germany, leaving Chancellor Angela Merkel out of options.

"One could sense that she was prepared to do things that cause a certain amount of griping in the group. One could recognise a bit of a gap between the chancellery and the finance ministry—something that has never been the case before," Norbert Barthle, a lawmaker from Merkel's Christian Democratic Union, told Wall Street Journal.