Swiss banks are shutting the accounts of Americans as the U.S. Internal Revenue Service accelerates the hunt for tax dodgers.

UBS AG and Credit Suisse Group AG, the country’s biggest banks, have told Americans to move their money into specially created units registered in the U.S., or lose their accounts. Smaller private banks such as Geneva-based Mirabaud & Cie. are closing all accounts held by U.S. taxpayers.

While the banks declined to say how many people are affected, more than 5 million Americans live abroad, including about 30,000 in Switzerland, according to estimates from American Citizens Abroad in Geneva. Swiss banks must register with the Securities and Exchange Commission to provide services for those customers.

“My bank doesn’t want to do that, so we wouldn’t accept an investment account for a U.S. person,” said Pierre Mirabaud, chairman of Mirabaud & Cie. and the Swiss Bankers Association, during a lunch at the American International Club of Geneva.