Prime Minister Alexis Tsipras launched a scathing attack on the International Monetary Fund for rejecting Greek reform proposals ahead of talks in Brussels Wednesday, denting hopes of a final debt deal to prevent Athens from defaulting and leaving the euro.

The leftist leader hit out at the "strange position" just minutes before going into an eleventh-hour meeting with key lenders including Christine Lagarde, the head of the IMF, and the European Union.

Markets dipped over fears that a meeting of eurozone finance ministers later Wednesday would fail to reach an agreement to unlock vital bailout funds that could be presented to an EU summit on Thursday.

"This strange position maybe hides two things: either they do not want an agreement or they are serving specific interests in Greece," Tsipras said.

"The repeated rejection of equivalent measures by certain institutions never occurred before — neither in Ireland nor Portugal," he tweeted, referring to bailouts in those two countries.

The EU-IMF creditors were due to present Tsipras with their own "common position" on Monday in response to the last-ditch list of reform proposals submitted by Athens on Sunday, sources said.

"It's up to the Greeks now to have their say," one source told AFP.

Tsipras — who recently accused the IMF of "criminal responsibility" for Greece's five years of austerity-driven woes — was meeting the key players from Greece's bailout monitors: Lagarde, European Commission President Jean-Claude Juncker and European Central Bank boss Mario Draghi.

"There is work to do," Jeroen Dijsselbloem, head of the Eurogroup of finance ministers from the 19-country single currency area, told reporters ahead of their meeting in Brussels which could stretch late into the night.

Wednesday's meetings had earlier been billed as potentially decisive, with the Greek government saying Tuesday it expected a deal within 48 hours.

Greece and its creditors have been locked in a stand-off since Tsipras was elected in January, the EU-IMF demanding reforms before unlocking the last 7.2 billion euros of Greece's bailout before it expires on June 30.

Athens needs to meet a a 1.5-billion-euro IMF loan repayment due on the same day and failure to pay means a default, with officials warning it could lead to Greece crashing out of the euro and even the European Union.

The new proposals by Greece aim to raise eight billion euros, mostly through new taxes on the wealthy and businesses, VAT increases and a cut in defense spending, officials said.

Athens insists the 'red line' of pensions remains largely untouched.

The IMF and Germany, Europe's biggest economy, are seen as being the most skeptical about the Greek proposals. Berlin is particularly keen to avoid any mention of debt relief in a deal.

Tsipras's comments seemed to be at odds with his Finance Minister Yanis Varoufakis, who said Wednesday said his country was in the "final stretch" of negotiations with creditors and could reach agreement shortly.

The Greek government meanwhile warned that any accord would have to be approved by a parliamentary majority before June 30, a tough task for Tsipras as many on the left wing of his party view him as reneging on campaign promises.

Tsipras was elected in January on the back of a promise to end half a decade of painful austerity imposed by two huge EU-IMF bailouts worth 240 billion euros, during which time Greece's economy has collapsed and unemployment has soared.

Thousands of protesters answered a call by unions close to Greece's communist party, turning out in central Athens Tuesday night to challenge the reforms.

The return of 23 percent VAT on the restaurant sector — up from the current 13 percent — has already been described as "the kiss of death" by the head of an association of restaurant chains, Thanassis Papanikolaou.

Any deal will also need to deal with what comes next, with EU officials suggesting an extension of the bailout until the end of the year followed by a possible third aid package to keep Greece afloat.

The two huge bailouts since the Greek crisis erupted in 2010 have left it with debt totaling nearly 180 percent of its annual economic output.