By Nektaria Stamouli in Athens and Bertrand Benoit and Andrea Thomas in Berlin 

Greece's Prime Minister Alexis Tsipras tried to convince voters he could quickly secure a better deal with creditors if his nation rebuffed their previous demands in Sunday's referendum, in an effort to sway Greeks who fear a "no" vote would instead mean exit from the euro.

In a television interview broadcast late Thursday, Mr. Tsipras stepped up his call for Greeks to reject lenders' terms in the referendum and said the country would have a bailout deal within 48 hours after the vote if they did.

"If 'no' wins, I can assure you that the next day I will be in Brussels and there will be a deal," he said.

Mr. Tsipras's pledge flies in the face of warnings from European creditors led by Germany, who say that a "no" vote would severely harm the chances for a deal and could increase the prospect of Greek bankruptcy and euro exit.

Complicating the picture, the International Monetary Fund warned in a review of Greece's financial outlook that the country needed a comprehensive debt restructuring by the eurozone, and additional bailout cash totaling more than EUR60 billion ($66.5 billion) through 2018, to return the country back to health.

The IMF's message: Greece's economic situation has considerably worsened thanks to the escalating conflict with creditors, and any new rescue deal that involves the fund will require greater financial generosity from Europe than it has been willing to countenance so far.

The referendum, which could determine whether the country leaves the euro or embarks on further difficult negotiations, is splitting the country and spreading dissent inside the Greek government as the country faces a potentially devastating bankruptcy. Eurozone finance ministers say there will be no further bailout talks until after the vote. Even if Greeks vote "yes," the road to a financial rescue for Greece could be long and painful.

A shorter version of the IMF's debt review, leaving out some of its bleaker predictions, is one of the two documents that Greeks are voting on in Sunday's referendum.

"The referendum is the last stop before the final deal," Mr. Tsipras promised voters, adding that "the bigger the 'no' the better the deal" his country would get.

Mr. Tsipras's view that he'd be able to strike a deal immediately after a "no" vote isn't shared elsewhere in Europe.

Officials from Germany and other creditors have said that Greece's next bailout program--if there is one--would require tougher measures than those Greeks would have rejected in a "no" vote, because the country's economic outlook is deteriorating due to its closed banks and capital controls.

Also, an entirely new application for a rescue from the eurozone bailout fund, the European Stability Mechanism, would now be needed because Greece's old bailout expired on Tuesday. Such an application would require evaluations and parliamentary debates in Germany even before new talks could start.

In addition, Mr. Tsipras is now seen in Berlin and other eurozone capitals as an unpredictable leader--especially since his call for a "no" vote against creditors' demands--and one who can't be relied upon to overhaul Greece's economy as a new bailout deal would require. Any new rescue program with the Tsipras government would be politically hard to sell in many creditor countries.

Greek Finance Minister Yanis Varoufakis, speaking on Bloomberg Television, expressed optimism that Greeks would follow the government's recommendation. He said he wouldn't stay on as finance minister if Greeks voted "yes" in the referendum, making clear the government's future hangs in the balance.

"I personally won't sign another extend and pretend," he said.

Protracted negotiations mean Greece might not get any more funding before July 20, when it must repay more than EUR3 billion in bonds held by the European Central Bank or face a second multibillion default on its debt within 20 days.

Greece became the first developed country to default on the IMF after it missed a $1.73 billion rescue-program payment on Tuesday.

Resistance to any more aid for Greece is growing in Germany's parliament, which must approve bailout financing.

Gunther Krichbaum, a lawmaker for German Chancellor Angela Merkel's Christian Democratic Union and head of parliament's European Union Affairs Committee, argued Greece wasn't even eligible for a fresh bailout program, which Athens requested on Tuesday.

To qualify for aid under the rules of eurozone's bailout fund, Greece would need to show that its debt is sustainable and that it only faces a liquidity crunch. In addition, failure to secure such aid would have to pose a danger for the financial stability of the entire eurozone.

Mr. Krichbaum said Greece satisfied none of these conditions since it had already defaulted to the IMF, and because most economists believe the ECB's bond-buying program insulates other eurozone members from the fallout of further Greek defaults.

Meanwhile in Greece, politicians from outside the governing left-wing Syriza party urged Greek voters to say "yes" to creditors' terms in the referendum, to safeguard the country's place in the eurozone.

Greek President Prokopis Pavlopoulos argued that the country's only option is Europe and the eurozone. "The referendum serves democracy only if it is taking place under conditions that don't allow division," Mr. Pavlopoulos said on Thursday. Former Greek prime ministers Costas Karamanlis and Costas Simitis--who oversaw the country's entry into the common currency--also threw their support behind the "yes" vote.

And in a sign of the bubbling dissent in the ruling coalition, five lawmakers from Syriza's junior coalition partner, Independent Greeks, distanced themselves from the government position on Thursday. Some said they intended to vote "yes" in the referendum, while others called on the government to cancel it.

Jeroen Dijsselbloem, the Dutch finance minister who presides over meetings of eurozone finance chiefs, said a "no" vote wouldn't entail Greece automatically leaving the common currency. But he stressed it would be an illusion for voters to believe that by voting "no," Greece would be in a position to negotiate a better bailout package.

Andrea Thomas in Berlin, Ian Talley in Washington and

Archie van Riemsdijk

in Amsterdam contributed to this article.

Write to Nektaria Stamouli at nektaria.stamouli@wsj.com, Bertrand Benoit at bertrand.benoit@wsj.com and Andrea Thomas at andrea.thomas@wsj.com