the creditors´ latest cash-for-reform plans would leave Greece "better armed" in the fight for a debt deal.
European Commission chief Jean-Claude Juncker set out a possible "last-minute" solution for an accord before the referendum, a commission source said
A euro-MP from Tsipras´s hard-left Syriza party said the plan should be studied.
Juncker told the Greek premier that a deal would involve accepting reform proposals that Greece´s EU-IMF creditors made at the weekend and backing a ´Yes´ vote in Sunday´s plebiscite. A Greek government source said that negotiations were taking place between Athens and its creditors, though a European Commission spokesman said time was "running out quickly".
German Chancellor Angela Merkel said she knew nothing about a new offer from Juncker for Greece.
EU leaders including Merkel, France´s Francois Hollande and Italy´s Matteo Renzi have joined Juncker in warning that the referendum would effectively be a vote on Greece´s place in the eurozone.
The government has called for Greeks to vote ´No´ rejecting the creditors´ debt proposals and Tsipras has staked his own career on the outcome.
Spanish Prime Minister Mariano Rajoy added Tuesday that a ´Yes´ victory in the referendum would allow Greece´s creditors to negotiate with a new government.
Athens lashed back Tuesday, with Labour Minister Panos Skourletis warning that European leaders want to "sink" Greece´s ruling Syriza party to block the rise of other far-left anti-austerity parties like its ally Podemos in Spain.
European leaders "fear the rise of forces like Podemos and they want to take the wind out of their sails through any means possible," Skourletis said in an interview.
He labelled Juncker´s call for Greeks to vote yes in the referendum as a "provocation".
"Today they interfere in the interior affairs of Greece, tomorrow they will do it in Spain and Italy," said Skourletis, a heavyweight in Greece´s coalition government.
Meanwhile a group of world-renowned economists including US Nobel Prize winner Joseph Stiglitz and leading French economist Thomas Piketty warned EU leaders against "creating bad history" in their bailout standoff with Greece.
In a letter published Tuesday in Britain´s Financial Times newspaper they called on creditors to give the country "a fresh start, bearing in mind, first, that the contractionary austerity policy demanded of Greece has been discredited by the IMF´s own research department..." while requiring the government to undertake needed reforms.
In Greece, many people have been caught up in lengthy queues at ATMs after banks were shut down for one week, to withdraw the maximum daily allowance of 60 euros ($66). Tourists are however allowed unlimited withdrawals.
"We brought quite a lot of cash with us, but we´ve been budgeting to make sure it doesn´t run out," said Michele Ammann, 48, from Switzerland.
"I feel sorry for the Greeks, they´ve been asked to sacrifice everything, right down to their underpants."
Many Greeks backed the government´s defiant stance against the country´s creditors, who they blame for forcing the country into years of painful recession by demanding swingeing austerity cuts.
"Our aim is for the referendum to be followed by negotiations for which we will be better armed," Tsipras has said, vowing to "respect the decision" of the Greek people.
As the tussle over Greece´s future intensified, sympathetic people donated over 30,000 euros to an online crowdfunding project set up by a British shoe-shop employee to help meet the IMF repayment
In more gloomy news for Athens, Standard & Poor´s ratings agency downgraded Greece´s credit assessment deeper into junk territory, saying the referendum brought it closer to default.
Fitch also cut its ratings on four major Greek banks to "restricted default".
Greece has debt worth nearly 180 percent of its GDP after receiving two bailouts worth 240 billion euros since 2010. Unemployment has more than doubled since 2009 to 25.6 percent and pensions and benefits roughly halved between 2010 and 2014.