The Greek government has submitted to creditors a new two-year aid proposal calling for parallel debt restructuring, the office of Prime Minister Alexis Tsipras said on Tuesday, in what seemed like a last-ditch effort by Athens to resolve an impasse with lenders.Greece also asked creditors to extend its current bailout program "for a short period of time" in order to avert a technical default until a new loan package is in force.

The statement came hours before Athens was set to default on a loan to the International Monetary Fund.

Eurozone finance ministers called a last-minute conference call to discuss the Greek request. 

But German Chancellor Angela Merkel told lawmakers Tuesday that Germany won't enter into new aid negotiations with Greece before its weekend referendum.

After months of wrangling and acrimony, the growing possibility that Athens could be forced out of the single currency brought into sharp focus the chaos that could be unleashed in Greece as well as the danger that would arise for the stability of the euro.

"What would happen if Greece came out of the euro? There would be a negative message that euro membership is reversible," said Spanish Prime Minister Mariano Rajoy.

"People may think that if one country can leave the euro, others could do so in the future. I think that is the most serious problem that could arise."

European Commission President Jean-Claude Juncker appealed to Athens to accept the deal proposed by international creditors last week while holding out hopes that some extra tweaks could still be possible.

Under Juncker's offer, Tsipras would have to send a written acceptance by Tuesday of the terms published by the EU executive on Sunday.

The last-ditch bid from Brussels came as uncertainty built ahead of Sunday's referendum, with a string of European leaders warning that it would effectively be a choice between remaining in the euro or reverting to the drachma.

Opinion polls show Greeks in favor of holding on to the euro but a rally of tens of thousands of anti-austerity protestors in Athens on Monday highlighted the defiance many in Greece feel about being pushed into a corner by the lenders.

Tsipras broke off negotiations with the Commission, the IMF and the European Central Bank and announced early a referendum on the bailout terms next Sunday.

French Finance Minister Michel Sapin, who has been most sympathetic to Athens in the negotiations, said in a television interview that negotiations could continue if Greeks voted "yes" on Sunday, but added: "With a 'no,' we go into an unknown territory."

As the hours ticked by before the bailout officially expires later on Tuesday, Greek officials have said the government will not make a 1.6 billion euro debt repayment to the IMF which is also due on the same day.

If that does not happen, IMF Managing Director Christine Lagarde will immediately report to the global lender's board at the close of business, Washington time, that Greece is "in arrears" - the official euphemism for default.

It will be the first time in IMF history that an advanced economy has defaulted on a loan from the world's financial backstop, putting Athens in the same bracket as Zimbabwe, Sudan and Cuba.

Greece has received nearly 240 billion euros in two EU/IMF bailouts since 2010. Leftist Finance Minister Yanis Varoufakis argues that Athens has not benefitted from the money, which largely went to repay German and French banks which had imprudently lent large sums to successive Greek governments.

The Greek economy has shrunk by more than 25 percent since 2009 and unemployment has soared to over 25 percent, including more than 50 percent of young job seekers.