The repercussions of a default would be hard to predict. But it could create a contagion of financial fear that could spread to other weak euro zone economies and force Greece to become the first nation to leave the 17-member euro currency union, a departure whose social and political ramifications might also defy prediction.

A key to securing the next bailout payment could be Greece’s reaching a new debt-revamping agreement with its private sector bondholders. Charles H. Dallara of the Institute of International Finance, the group representing the private creditors, was to meet Thursday evening in Athens with the Greek prime minister, Lucas D. Papademos. There was no word about the status of those talks by late Thursday.

Negotiations between the two sides have foundered twice already over disagreement on how much loss private investors should be willing to absorb on bonds.

As Greece’s woes have escalated, so have its demands on the amount of loss the creditors should accept. While creditors have said they would be willing to accept a loss of 70 percent on their new bonds, Greece and its backers have been pushing for more by demanding that these securities carry an interest rate below 3.5 percent.

Greece is effectively bankrupt, staggering under a debt load that the I.M.F. now estimates as equal to about 160 percent of its gross product, with an economy so weak the government can no longer meet debt payments on its own. That is why it is to receive as much as 130 billion euros ($169 billion) in bailout money under an agreement struck last October with the so-called troika: the European Union, the European Central Bank and the I.M.F.