The Greek prime minister has vowed to tackle the country's fiscal crisis
Greece's Prime Minister George Papandreou has insisted that his country is "not about to default on its debts".
He also said it was "out of the question" to resort to an International Monetary Fund loan.
Greece has said its debt stands at 300bn euros ($442bn; £272bn), the highest level in its modern history.
Greek government bonds are having their worst week in at least 11 years, after its debt rating was lowered this week.
"We recognise that the problems are serious, that the challenge is huge," said Mr Papandreou, who is expected to make a statement on 14 December to outline plans to reassure international creditors.
He has pledged to reform the debt-ridden economy, whose public deficit is expected to surge to 12.7% of output this year.
Greece's total debt amounts to 113% of its gross domestic product.
This week, Greece's debt rating was cut by the the ratings agency Fitch to BBB+, the lowest of any of the 16 nations that have the euro.
Investors have sold Greece's two-year government bonds heavily this week, sending bond yields up the most since at least 1998.
When bonds are sold, the price drops and the yield - the amount the bond pays to its owner - rises.
Investors are worried that Greece may not be able to pay off its huge debts, and are seeking safer countries for their money.
This week two-year bond yields have surged to 3.09% from 1.9%.
Ten-year Greek bonds had their worst weekly decline since January, with the yield up to 5.3% from 4.99%.
By contrast, the German 10-year bund - which is the benchmark government bond for Europe - yields 3.2% and the two-year bond pays only 1.2%.
The current difference between Greek and German bonds is four times more than what it has usually been over the past decade, indicating just how risky investors think it is to hold Greek debt.