Credit rating agency Moody's has downgraded two French banks because of their exposure to Greek debt.
A third bank is on review for a possible downgrade.
German Chancellor Angela Merkel and French President Nicolas Sarkozy are due to hold talks with Greek Prime Minister George Papandreou, in response to growing market fears of an imminent debt default by Greece.
The markets for short-term cash lending between European banks have become increasingly stressed in recent days, while share prices in European banks have fallen sharply.
In a sign of the international alarm over the crisis, China and the United States have urged Europe's leaders to prevent the euro area debt mess, now threatening Italy, from spreading.