Finance minister George Papaconstantinou says Greece is not at risk of default
Investors are still nervous over plans to rescue the Greek economy, despite indications of progress on bail-out talks over the weekend.
Interest rates demanded by investors lending money to Greece are close to record highs - a sign that fears remain over Greece's ability to repay loans.
Greece has been forced to request 40bn euros from the EU and International Monetary Fund (IMF).
But the details of the bail-out deal have not yet been finalised.
There is also concern that European partners in the deal - particularly Germany - are reluctant to participate in the bail-out.
Time running out
On Monday, Greek bond yields - the interest rate that Greece must pay to borrow money on the international markets - were just below the 9.15% record seen last week.
The interest rate is 6.14 percentage points higher than that charged to Germany - seen as the safest investment in Europe.
That is the largest spread in bond yields for 12 years.
Greek Finance Minister George Papaconstantinou said talks with the IMF over the weekend had progressed well, with the head of the IMF, Dominique Strauss-Kahn, saying that a deal would be agreed "in time to meet Greece's needs".
Negotiations must stick to a series of deadlines, with 8.5bn euros needed before 19 May, when Greece faces its next bond repayment.
The IMF is expected to provide 10bn euros of support this year, with eurozone nations providing a further 30bn euros. But the scale of assistance after this year remains unclear.
That could affect the willingness of European countries to commit to a bailout - another source of uncertainty for investors.
European leaders want to attach strict conditions to any bailout
The Netherlands has already said it will wait to hear full details of the plan before putting the necessary legislation before its parliament.
It is among those European countries looking for further austerity measures from Greece before they commit to bailout money.
Senior officials in Germany's governing coalition have indicated that they are willing to help Greece as long as "strict conditions" are met, despite describing financial help as a "last resort".
French Finance Minister Christine Lagarde has also pledged to "hold Greece accountable" for any financial help.
However, demands for even harsher cuts in Greece would almost certainly be met with domestic opposition.
There have already been public demonstrations against proposed cuts to public spending that the government says are necessary to bring down the country's massive debt mountain.
Greece's budget deficit was equivalent to 13.6% of GDP last year - more than was estimated by finance ministers.
Currently, ambitious government plans are to bring this down to 3% by the end of 2012.
Despite the uncertainty, Mr Papaconstantinou has sought to reassure investors by denying that Greece is in danger of defaulting on its debts, pointing out that short-term bridging loans could be used should the IMF deal falter.
He also warned speculators betting on a Greek default.
"All I can say is that they will lose their shirts," he told reporters over the weekend.