The U.S. loses about $100 billion annually due to offshore tax evasion, according to a Senate probe that is taking aim at Swiss bank UBAG and Liechtenstein's LGT Group for allegedly marketing tax-evasion strategies to wealthy Americans.

U.S. clients hold about 19,000 accounts at UBS, with an estimated $18 billion to $20 billion in assets in Switzerland, according to the findings from the Senate probe and Justice Department prosecutors.

The probe adds fuel to a burgeoning effort by tax authorities around the globe to shatter the veil of bank secrecy that tax havens hide behind in catering to the world's elite.

Investigators were not able to obtain data about LGT but said the IRS has identified at least 100 accounts with U.S. clients at the Liechtenstein bank.

Documents highlighted in the Senate probe include one LGT memorandum that describes a client using accounts to move funds "to the USA and Panama and may be classified as bribes.”

For investigators and prosecutors, the biggest break has come from testimony given by former employees of UBS and LGT. UBS banker, Bradley Birkenfeld, pleaded guilty to helping his U.S. clients evade taxes. He told U.S. prosecutors that the Swiss bank generates some $200 million a year in revenue from US. clients.

UBS is in talks with the IRS and the Justice Department. Authorities are having a tougher time seeking help from Liechtenstein, which is clinging to its secrecy laws. A UBS spokeswoman said the company continues to work with authorities to correct any improprieties found by the investigation.