The bank told Swiss news wire ATS that it had been informed by regulator FINMA that it was the target of investigations.
Just two weeks ago, Jörg Müller-Ganz, a new member of the bank's board, said he was confident that the bank would not be targetted by the US justice department.
But Swiss daily Tages-Anzeiger said that the bank was one of several Swiss institutions targetted by US tax authorities. Others include Credit Suisse, which has already confirmed that it was being probed, as well as Bank Wegelin and Julius Bär.
The United States has been putting pressure on Switzerland to ease its bank secrecy laws which Washington claims help to shield tax cheats.
In an offensive against Switzerland's biggest bank UBS in 2008 and 2009, US tax authorities managed to pry 300 client names and a fine of $780 million from the bank.
Dissatisfied, Washington then filed a second suit, which finally forced UBS to hand over data on 4,450 American clients.