The latest round of talks between Greek and EU officials in Brussels has failed to reach an agreement.
A European Commission spokesman said while that progress was made on Sunday, "significant gaps" remained.
Europe wants Greece to make spending cuts worth €2bn (£1.44bn), to secure a deal that will unlock bailout funds.
Greek deputy prime minister Yannis Dragasakis said that Athens was still ready to negotiate with its lenders.
He said Greek government proposals submitted on Sunday had fully covered the fiscal deficit as demanded.
However, Mr Dragasakis added that the EU and IMF still wanted Greece to cut pensions - something Athens has said it would never accept.
IMF chief economist Olivier Blanchard said in a blog post that an agreement will require "difficult decisions", with "tough choices and tough commitments to be made on both sides".
The cash-strapped nation is trying to agree a funding deal with the European Union and IMF before the end of June to avoid a default.
Eurozone finance ministers will discuss Greece when they meet on Thursday. The gathering is regarded as Greece's last chance to strike a deal.
The Commission spokesman said: "President [Jean-Claude] Juncker remains convinced that with stronger reform efforts on the Greek side and political will on all sides, a solution can still be found before the end of the month."
The talks come as Germany ramps up pressure on Greece. Vice-chancellor Sigmar Gabriel said on Sunday that European nations were losing patience with Greece.
Germany wanted to keep Greece in the eurozone, but writing in Bild he warned that "not only is time running out, but so too is patience across Europe".
Mr Sigmar is also economy minister and head of junior coalition partners the Social Democrats.
His article is seen as a warning, particularly as his party has been more sympathetic to Greece in the past.
"Everywhere in Europe, the sentiment is growing that enough is enough," he wrote.
Greece is seeking to avoid defaulting on a €1.5bn debt repayment to the IMF due by the end of the month.
Creditors have demanded cuts in spending in return for another tranche of bailout funds.
But Greece's ruling left-wing Syriza party, led by Alexis Tsipras, was elected in January on promises to ease up on the highly unpopular austerity measures, increase the minimum monthly wage and create more jobs.
However, on Saturday Mr Tsipras warned the Greek people to prepare for a "difficult compromise".
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