In a speech in Rome Visco, who is governor of the Bank of Italy, said the outcome of negotiations between Athens and its creditors remained "profoundly uncertain" after inconclusive talks in Brussels on Tuesday.

Visco said the ECB, which has been providing cash-strapped Greek banks with emergency liquidity, could not continue to support them for long without progress on a cash-for-reforms deal at the political level.

"To maintain support it is necessary that there is a clear prospect of a political agreement on a programme," he said.

Visco said the ECB's bond-buying programme known as quantitative easing (QE) had not so far been significantly weakened by the Greek crisis and the bank was "determined to "fully carry out" the programme to increase inflation.

"Any unwanted restrictions in monetary conditions will be strongly countered," he said.

"The Governing Council is closely watching developments on financial markets and their implications for monetary conditions and price stability in the euro zone; it is ready to use all instruments at its disposal," Visco said.

Without mentioning the risk of Greece leaving the euro zone which many analysts now consider probable, Visco said the direct effects of the crisis on the euro zone were likely to be modest.

"However the crisis could have more serious repercussions if it rekindles the fear among international investors that the euro zone is not irreversible," he said.

"In this case coordinated action by national and European authorities (to counter market tensions) will be indispensable."

(Reporting by Stefano Bernabei and Giuseppe Fonte, writing by Gavin Jones)