Investors are increasingly nervous about Greece's ability to repay its debts.
Bonds issued by Greece are changing hands for ever-lower prices as they are seen to be less secure.
It that loss of confidence continues, it could become a self-fulfilling prophecy, forcing Greece to seek help.
But Prime Minister George Papandreou has repeatedly denied speculation that Greece will have to be bailed out.
The BBC reports Greek government debt is considered the riskiest since the euro was formed in 1999.
Ten-year bonds cost 3.96 percentage points more than German debt on Thursday, the highest since 1998.
Greece's public debt stands at about 300 billion euros ($US419 billion).
The European Union says there's no chance of Greece defaulting or leaving the eurozone.
Greece has already received technical assistance from the International Monetary Fund, which says there is no need to bail Greece out.
