Mrs Merkel says the priority is for Greece to put its house in order
France and Germany have reached a deal on a financing plan to help debt-laden Greece, which will include IMF help.
Officials in Brussels say the package - yet to be agreed by other eurozone states - will total 23bn euros (£21bn).
The French presidency said there would be "very precise conditions" under which the 16 eurozone countries "could be led to intervene" to help Greece.
Co-ordinated bilateral loans and IMF loans are envisaged. EU leaders are now poised to discuss the plan.
News of the deal broke as leaders of the 27 EU member states gathered for a two-day summit in Brussels.
Diplomats say the eurozone leaders may hold an additional meeting on Thursday evening to discuss how to help Greece.
So far the eurozone has avoided seeking an IMF loan for Greece, preferring a European solution and anxious to maintain global confidence in the euro.
Revision of rules?
Earlier on Thursday Germany's Chancellor Angela Merkel said the German government "will press for emergency aid combining IMF and joint bilateral aid from the eurozone but... only as a last resort".
She has signalled reluctance to offer Greece anything resembling a bail-out, which is not allowed under the single currency rules.
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.
Stressing the need to learn lessons from the crisis, she wants a treaty change to allow sanctions to come into force should a eurozone country ever default on its debts.
Arriving at the summit, Greek Prime Minister George Papandreou urged EU leaders to act to stabilise the euro.
The euro hit a 10-month low against the dollar on Wednesday after a credit downgrade for Portugal, which is also struggling with heavy debts.
Greek austerity drive
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.
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.