The chairman of a meeting of Eurozone finance ministers says fresh conditions will be attached to a 130bn euro ($170bn; £110bn) bailout for Greece.
Jean-Claude Juncker said the Greek parliament would now have to pass a package of cuts and reforms on Sunday.
In addition, Greek politicians will have to find an extra 325 million euros ($432m; £273m) in savings for 2012.
Greek unions had already called a 48-hour strike, beginning on Friday, in opposition to the measures.
Mr Juncker, the prime minister of Luxembourg, also said Greek political leaders would have to give "strong political assurances" that they will continue to implement reforms after a general election due in April.
"Despite the important progress achieved over the last days, we did not yet have all necessary elements on the table to take decisions today," he said, referring to a plan agreed by Greece's fragile ruling coalition after days of talks.
"All these measures are important to ensure a smooth implementation of the programme also after the upcoming general elections.
"These three elements, those I mentioned, need to be in place before we can take decisions," Mr Juncker added.
He further welcomed "assurances provided by the Greek government that all the necessary elements will be put in place in the coming days," Reuters reports.
The conditions would need to be fulfilled before Eurozone finance ministers reconvene on Wednesday, he said.
BBC