FRANKFURT — French and German banks have lent nearly $1 trillion to the most troubled European countries and are more exposed to the debt crisis than the banks of any other countries, according to a new report that is likely to add pressure on institutions to detail their holdings.

French banks had lent $493 billion to Spain, Greece, Portugal and Ireland by the end of 2009 while German banks had lent $465 billion, according to the report by the Bank for International Settlements, an institution based in Basel, Switzerland, that acts as a clearing house for the world’s central banks.

The report sheds light on where the risks from Spain and other troubled euro-zone countries are concentrated, but left open the question of which individual banks would be most endangered by declines in the prices of sovereign bonds or a surge in bad loans made to companies and individuals. The B.I.S. did not identify individual institutions, in line with its confidentiality rules.

Voluntary disclosure by banks has been uneven. Hypo Real Estate Holding, a real estate and public-sector lender based near Munich, has put its exposure to government debt from the four countries plus Italy at more than €80 billion, or $97 billion. Deutsche Bank, in Frankfurt, says it holds €500 million in Greek government bonds and no Spanish or Portuguese sovereign debt.