BRUSSELS—The International Monetary Fund is working with national authorities in southeastern Europe on contingency plans for a Greek default, a senior fund official said—a rare public admission that regulators are preparing for the potential failure to agree on continued aid for Athens.

Greek banks are big players in some of its neighbors’ financial systems. In Bulgaria, subsidiaries of National Bank of Greece SA, Alpha Bank SA, Piraeus Bank SA and Eurobank Ergasias SA own around 22% of banking assets, roughly the same as...