The situation in Greece is not formally on the summit agenda
Chancellor Angela Merkel has told Germany's parliament she will insist that the IMF is involved if debt-ridden Greece needs to be bailed out.
Before heading to an EU summit in Brussels, she made clear she would disappoint other countries like France, which want a purely EU bail-out.
Mrs Merkel also said she would seek EU treaty changes to stop future crises.
Arriving at the summit, Greek Prime Minister George Papandreou urged EU leaders to act to stabilise the euro.
'Strengthen the eurozone'
The EU's single currency hit a 10-month low against the dollar on Wednesday after a credit downgrade for Portugal, which is also struggling with heavy debts.
The aim should be to ensure "we strengthen the eurozone, we strengthen the euro and we stabilise the euro currency," Mr Papandreou told reporters.
Germany blocked a pre-summit meeting of eurozone leaders to discuss the Greek crisis.
Berlin apparently believes there is no point in such a meeting because a deal is not imminent. But diplomats said such a meeting could still take place on Thursday evening after the close of official business for the day.
Mrs Merkel fears that Germany would end up footing the bill if the EU funded the bail-out package itself.
She wants the IMF to take the lead in a joint package with the EU, and tough sanctions for countries that break budget rules.
Before heading to Brussels on Thursday she told the German parliament: "The German government will press for emergency aid combining IMF and joint bilateral aid from the eurozone but I say again, only as a last resort".
She said Greece was not insolvent, was acting to curb its deficit and was still able to raise money on international markets.
She also said she would press for the EU to amend its treaties to strengthen its ability to prevent future budget crises.
Angela Merkel does not want Germany to foot the bill for a bail-out
EU members Hungary, Latvia and Romania have received emergency loans from the IMF and EU as their budgets have been hit hard by the global economic downturn. But, unlike Greece, they are not in the eurozone.
The Greek crisis is not formally on the agenda of the summit, which is officially concerned with the EU's 10-year economic strategy, and reinvigorating international negotiations over global warming.
But, says the BBC's Oana Lungescu in Brussels, it is Greece that is on everybody's mind.
European Commission President Jose Manuel Barroso said it would be difficult to avoid the subject, and called on EU leaders to signal their commitment to the euro by reaching agreement on "a safety net" for Greece.
"What is at stake is the essential principle of financial stability that is at the centre of the euro, and the euro is the most important creation of the European project," he told the European Parliament on Wednesday.
Mr Barroso is a former prime minister of Portugal, whose credit downgrade on Wednesday heightened fears that the Greek crisis is starting to spread.
Some countries believe that only a swift and credible deal among eurozone nations can stem the uncertainty and restore faith in the euro.
To that end, France and Spain called for a last-minute meeting of eurozone leaders before the summit, which is due to get under way shortly.
German taxpayers are fiercely opposed to bailing out Greece, which is burdened by debt of nearly 300bn euros (£267bn, $407bn) and a public deficit of 12.7% of GDP - more than four times the official eurozone limit.
Greece has enacted unpopular measures to curb its deficit, and has signalled it is ready to turn to the IMF for assistance.
It is also having to refinance its debt. Because of doubts over its ability to pay, it is having to pay interest at about 6% - around double what Germany has to pay.