She told a press conference in Berne that "no data on clients of the banks has been transmitted to the United States." For Switzerland, any exchange of information on client data is only possible if it is based on existing legislation," she said, referring to the double taxation treaty reached in 2009 with the US.

Calmy-Rey refused to give more information on the subject. Local media outlet SonntagsZeitung reported earlier this week that Washington had asked for detailed information on US nationals who might have improperly hidden money in Switzerland, basing its report on a purported letter from the US deputy attorney general James Cole dated August 31, addressed to Swiss authorities.

The letter mentioned Switzerland's second-biggest bank, Credit Suisse, as well as about 10 banks other banks, notably Julius Baer, Wegelin, and the cantonal banks of Zurich and Basel, the Sunday paper alleged.

US authorities want all data concerning private customers and US foundations which have deposited at least $50,000 (35,300 euros) in Switzerland between the period of 2002 and July 2010, it said.

The Swiss Financial Market Supervisory Authority surveyed a number of Swiss banks and determined that between 20 billion to 30 billion dollars (14.2-21.3 billion euros) was held by "tens of thousands" of US customers, another media outlet, the TagesAnzeiger, reported on Tuesday.

This latest request is not the first by US officials. Switzerland's biggest bank UBS was forced to disclose the names of 4,450 US clients for whom it had offered to conceal funds from the eyes of the US tax inspectors. The bank paid a fine of 780 million dollars to avoid losing its banking licence in the United States.