"The letter exists," a spokesman from the bank told AFP, referring to media reports of a statement sent by the bank to its US clients over the tax investigation.
He declined to say how many of the bank's clients are recipients of the letter, which also advises the clients to engage a contact person in Switzerland who could represent them if required at the Swiss tax office.
Washington has formally asked Bern to hand over details of Credit Suisse's US clients, as part of its probe into whether 11 Swiss banks helped US citizens to evade taxes.
Under the procedure, the Swiss government would have to decide whether to agree to Washington's request.
The client in question also has the right to appeal against Bern's decision.
In its third-quarter earnings report, Credit Suisse said it had set aside 295 million francs ($327 million) in provisions for the case.
The widened probe against Swiss banks follows a successful offensive against Switzerland's biggest bank UBS, which was forced to pay $780 million in fine and to send details of 4,000 clients to US tax authorities.