Italian President Sergio Mattarella said on Thursday he hoped a "balanced" deal could be found allowing Greece to solve its economic crisis without leaving the single European currency or the European Union.

"We hope that Greece might quickly find a balanced agreement for resuming a path of stability and growth within the fold of the European Union, to which Athens belongs," Matarella said.

Higher interest rates could cost Italy an extra 11 billion euros (around 12.2 billion euros) to service its debt -- the most in the eurozone -- should Greece leave the eurozone, ratings agency Standard & Poor's forecast on Thursday.

A Grexit could cost the eurozone as a whole an additional 30 billion euros in 2015-2016, Standard & Poor's said.

Greece's leftwing Syriza government, elected on an anti-austerity platform, has been in deadlock with its creditors for months over the terms of a third bailout of the debt-laden country.

On Sunday, Greek voters are being invited to say yes or no to the terms of a third bailout in a referendum.

The previous eurozone bailout expired on Tuesday denying Greece access to billions of euros in funds, and Athens missed a $1.7-billion repayment to the International Monetary Fund (IMF).

The country's banks have been closed to all but pensioners for the past four days and the country's media are reporting growing pressure on businesses.