Greece demanded "major concessions" from its creditors Monday after Prime Minister Alexis Tsipras accused them of looting the country after last-ditch talks to avert a calamitous default collapsed.

With the Athens stock market diving, Tsipras said Greece would "wait patiently" until its creditors — the International Monetary Fund and the European Union — become "more realistic."

He lambasted the creditors for "political opportunism" in trying to force Athens to take an axe to its pension system, a concession the leftist anti-austerity government has steadfastly refused to make.

"One can only see a political purposefulness in the insistence of creditors on new cuts in pensions after five years of looting under the bailouts," Tsipras told Greek newspaper Efimerida Ton Syntakton.

But Athens immediately came under intense fire from its EU partners, with a German finance ministry spokesman saying "the ball is in Greece's court" and French President Francois Hollande urging Athens not to "waste time."

"This message is for Greece," Hollande told reporters at the Paris Air Show. "It should not wait and should restart discussions with the institutions... Let's not waste time."

European Commission Vice-President Jyrki Katainen, for his part, said: "A solution is possible but this depends on the Greek government.... We continue to hope."

Quoted by the Austrian daily Der Standard, he said: "There is a lot of goodwill... You can always provide more liquidity but this is nothing without structural reforms."

Greek officials Sunday blamed the failure of the latest round of talks on the IMF, the country's most hardline creditor. "The demands of the creditors are irrational," an irate Greek government source said.

The Athens Stock Exchange plunged 7.14 percent in early trade Monday before recovering to a 4.68 percent drop at midday, with bank stocks especially hard hit.

Greece's main market was on a rollercoaster last week, losing 5.92 percent on Friday on news that the fed-up eurozone nations were making contingency plans for a Greek default, after rallying 8.16 percent just the day before.

"In light of Greece being unwilling or unable to make any concessions at all... it remains doubtful that a solution to avoid a Greek default can be reached," analyst Markus Huber at London brokerage Peregrine & Black said Monday.

The talks have aimed to break a five-month standoff between Athens and its creditor overseers, who are demanding reforms in return for the last 7.2 billion euros ($8.1 billion) of Greece's 240-billion-euro bailout.

The bailout expires on June 30, and to meet that deadline, a reform deal must be resolved by a meeting in Luxembourg on Thursday of the eurozone's 19 finance ministers, who control the bailout purse strings.

Also at the end of the month, Greece faces a huge 1.6 billion-euro payment to the IMF with another 6.7 billion euros due to the European Central Bank in July and August, which Greek officials have said the government cannot afford.

Valdis Dombrovskis, the EU's vice president responsible for the euro, said Monday that in view of the "persistent uncertainty," the EU had slashed its projection of Greece's GDP growth for 2015 from 2.5 percent to 0.5 percent.

Speaking at the start of Innovative Enterprise Week in Riga, he said: "Needless to say, all projections for Greece are subject to a particularly high degree of uncertainty since... the agreement is still not reached."

All sides had agreed that the talks were the last chance for Athens to unlock vital bailout cash in return for tough reforms that Tsipras still doggedly refuses.

The IMF's position was "intransigent and tough" because it was insisting on further pension cuts and a rise in value-added tax on basic goods, like electricity, the Athens source added.

But in a rare statement on their position in the talks, the IMF took a conciliatory approach, writing in an official blog that a deal would require "difficult decisions by all sides" -- including Greece's European partners.

Greece is shattered economically after six years of crisis and despite two rescue programmes since 2010.

Its debt mountain is equivalent to 180 percent of GDP, or almost twice the country's annual economic output.

According to an EU source, savings from the reform measures put on the table by Greece fell short by two billion euros.

Greek Finance Minister Yanis Varoufakis told Germany's Bild newspaper in an interview published Monday that a deal was still possible and could even be concluded "in one night" if German Chancellor Angela Merkel were in attendance.

But any deal also needs to go to many national parliaments, including in Germany, where passage is not certain.