Switzerland has strict legislation on bank secrecy
Swiss bank UBS has refused a US government demand to provide information on 52,000 US clients.
The request was made in a lawsuit filed earlier in the day in Miami as part of a tax fraud investigation.
The Obama administration wants UBS to turn over information on American customers who hid accounts from US authorities, in violation of tax laws.
A deal on Wednesday provides access to about 250 to 300 UBS customers who used Swiss bank secrecy laws to hide assets.
"At a time when millions of Americans are losing their jobs, their homes, and their health care, it is appalling that more than 50,000 of the wealthiest among us have actively sought to evade their civil and legal duty to pay taxes," the acting assistant attorney general, John DiCicco, said in a statement.
Bank chairman Peter Kurer said UBS accepted "full responsibility" for helping its US clients hide assets from the Internal Revenue Service.
But the bank said it had a defence against the enforcement of the summons to hand over account details, and that it would vigorously contest the enforcement of the summons in the civil proceeding.
On Wednesday UBS agreed to pay $780m (£549m) to the US government to settle allegations that it had defrauded US tax authorities, after being accused of conspiring to create sham accounts to hide clients' assets.
Switzerland has strict bank secrecy laws but the country's financial regulator ordered UBS to hand over some client data to the US government.