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

UBS and Credit Suisse Group, the country's biggest banks, have told Americans to move their money into special units registered in the United States, or lose their accounts.

Smaller private banks such as Geneva-based Mirabaud & Cie are closing all accounts held by US taxpayers.

More than five million Americans live abroad, including about 30,000 in Switzerland, estimates from American Citizens Abroad in Geneva show.

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 US person," said Pierre Mirabaud, chairman of Mirabaud & Cie and the Swiss Bankers Association.

SEC registration means clients don't enjoy the protection of Swiss banking laws, which make it a crime for money managers to disclose the names of clients without their consent.

Switzerland said in March it would co-operate with international tax evasion probes after Zurich-based UBS admitted helping US clients avoid taxes.

The IRS has since increased pressure on Americans to disclose offshore accounts as it seeks to recoup an estimated US$50 billion ($77.7 billion) in unpaid taxes. The agency set a deadline of September 23 for taxpayers to declare all foreign accounts or face possible criminal prosecution that could result in as much as 10 years in prison and $500,000 in penalties.

The US has also proposed increasing reporting and oversight requirements for so-called qualified intermediaries - foreign banks that withhold taxes on behalf of the IRS. That may increase the cost of compliance and the risk of violating US laws, said Charles Adams, managing partner at law firm Hogan & Hartson in Geneva.

"American citizens are starting to feel like they're Typhoid Mary," said Adams. "The Swiss simply don't want American customers because it requires so much infrastructure and hassle that they don't make any money."

Sandra Dysli, an American who has lived in Geneva for 40 years, said Bank Zweiplus, the Zurich-based joint venture of Basel-based Bank Sarasin & Cie and AIG Private Bank, and a Geneva branch of Raiffaisen International Bank-Holding refused to open investment accounts for her.

"I was told that I cannot legally be a client because I'm an American," said Dysli, who retired from the United Nations in 2001. "I couldn't get an investment account and had everything in cash."