Arnd Wiegmann/ReutersA branch of Credit Suisse in Basel, Switzerland.

11:31 a.m. | Updated

PARIS — Credit Suisse has been ordered by the Swiss government to turn over account data on some wealthy American clients as part of a U.S. effort to crack down on tax evasion, the bank said on Tuesday.

The bank, based in Zurich, wrote in an e-mailed statement that the U.S. Internal Revenue Service had recently asked the Swiss Federal Tax Administration for help in locating information on American account holders under a 1996 American-Swiss tax treaty.

Credit Suisse said the Swiss tax administration had responded with ‘‘an order directing Credit Suisse A.G. to submit responsive account information’’ to the Swiss authorities.

Alex Biscaro, a Credit Suisse spokesman in Zurich, said the bank had begun to inform some U.S. clients by letter about the order, but he declined to comment further on the case. Beat Furrer, a spokesman for the Swiss tax administration, declined to detail the nature of the request. Dean Patterson, a spokesman in Washington for the I.R.S., declined to comment.

The letters that Credit Suisse was sending to clients gave two options, according to Paul Behling, a partner at Withers Bergman, an international law firm: Either consent to the account data being turned over to the I.R.S. or file an appeal with the Swiss authorities.

Mr. Behling said he would advise clients who believed they had a basis to appeal to do so, but that others should consider going to the I.R.S. and trying to negotiate a lighter penalty. Under U.S. law, cheating the tax authorities can be punished with up to five years in prison and civil penalties.

Not all of the bank’s U.S. clients are affected by the order. The I.R.S. provided Swiss authorities with detailed information on the Credit Suisse clients in question, suggesting they had obtained information about those individuals independently. The fact that the request was made under the existing treaty showed that there has been no global deal on client data.

‘‘U.S. officials are mining the data from the 30,000 people who have participated in the voluntary disclosure programs,’’ Mr. Behling said, referring to an I.R.S. initiative to encourage people with hidden offshore accounts to come clean. Those who entered the program did so were required to name names about the bankers and other advisers who helped them to set up accounts and offshore corporations.

‘‘The initial focus of the I.R.S. on Credit Suisse seems to be on U.S. persons holding offshore accounts through corporations or trusts,’’ he said. ‘‘This is not the end of it.’’

The United States and several European countries, notably Germany, Britain and France, have been seeking in recent years to ensure that their citizens cannot take advantage of Swiss banking secrecy to hide assets. The Offshore Compliance Initiative, a U.S. Justice Department effort to track down tax cheats, is conducting criminal investigations into at least eight banks.

The Justice Department told Credit Suisse in July that it was the object of an investigation as part of ‘‘a broader industry inquiry’’ after four private bankers with links to Credit Suisse were indicted in February by the U.S. authorities on charges that they helped Americans to avoid taxes.

Credit Suisse said last week that it was setting aside 478 million Swiss francs, or $535 million, for legal costs related to tax evasion charges in the United States and Germany. In September, it reached a deal with the German authorities to end an investigation over allegations that employees in Düsseldorf had helped German clients to hide income from tax collectors.

UBS, the biggest Swiss bank, paid $780 million in 2009 in the tax investigation and later agreed to hand over some client names to avoid prosecution.

In October, Bank Julius Baer, a private Swiss bank, said two of its advisers had been charged with conspiring to help clients evade U.S. taxes on more than $600 million hidden in offshore accounts.

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