Greece needs 50 billion euros over the next three years, including 36 billion euros more from EU lenders, to stabilize its finances even under existing creditor plans.
Greek Prime Minister Alexis Tsipras vowed to press ahead with a controversial bailout referendum as European leaders ruled out any fresh debt offer before Sunday's vote.
The International Monetary Fund said that allowing a borrower to delay repayment, as Greece has requested, is generally ineffective in helping a country overcome crisis.
Ukraine announced the immediate suspension of all Russian gas purchases after EU-mediated negotiations aimed at keeping supplies running for at least three more months broke down.
Nobody in the halls of the International Monetary Fund in Washington has any illusion: Greece is going to default, delivering a new blow to the global crisis bank's credibility.
Irish Prime Minister Enda Kenny said that Greece can still return to negotiations with its international creditors to avert a default and possible exit from the euro.
Russian Foreign Minister Sergei Lavrov expressed the hope that Brussels would avoid "negative scenarios" over Greece amid fears that the country is heading for a eurozone exit.
Brazil's state oil giant Petrobras slashed its five-year investment plan by a whopping 77 billion dollars -- as it tries to recover from a massive corruption scandal.
It has already been dubbed "Black Monday" -- jittery housewives, shoppers and business owners queued in vain at cash machines in Athens, where the country awoke to capital controls and shuttered banks.
The Greek government said it had called an emergency gathering of its systemic stability council, as speculation mounted of capital controls being imposed and increasing signs of a bank run.
Christine Lagarde has said she is ready to consider a second term as head of the International Monetary Fund in an interview with a French magazine to be published.
Crisis-hit Greece is in the "final stretch" of negotiations with creditors and could reach agreement shortly, Finance Minister Yanis Varoufakis indicated.
Iran's supreme leader Ayatollah Ali Khamenei restated his country's red lines for a nuclear deal in a dramatic intervention during a meeting with President Hassan Rouhani and top officials.
French Prime Minister Manuel Valls said that there is "still a lot of work to do" if Greece and its creditors are to reach an agreement to avert a possible default.