The International Monetary Fund dramatically raised the stakes in Greece's stalled debt talks on Thursday, announcing that its delegation had broken off negotiations in Brussels and flown home because of major differences with Athens.

The surprise IMF announcement came as the European Union told leftist Greek Prime Minister Alexis Tsipras bluntly to stop gambling with his cash-strapped country's future and take crucial decisions needed to avert a devastating default.

A Greek source told Reuters that the entire Greek delegation that had been negotiating a cash-for-reform deal had left Brussels for home on Thursday, citing outstanding disagreements.

"There are major differences between us in most key areas," IMF spokesman Gerry Rice told reporters in Washington. "There has been no progress in narrowing these differences recently and thus we are well away from an agreement."

Greece needs a deal to unlock aid before the end of the month