ZURICH-- Five state-backed Swiss banks said Monday they would participate in a U.S. disclosure program aimed at lenders suspected of having aided tax evasion by Americans, adding to the list of Swiss financial institutions agreeing to share information with U.S. authorities.

The cantonal banks of Vaud, St Gallen, Graubunden, Lucerne and Zug said Monday they will take part in the joint program between the U.S. Department of Justice and Swiss officials that allows firms to identify any undeclared U.S. assets on their books.

Banks found to have undeclared U.S. assets will likely pay fines based on the amounts and length of time they've been on the banks' books.

The five cantonal banks follow seven other banks, including state-backed Berner Kantonalbank and PostFinance, which declared their intention to participate in the scheme last week.

Two other cantonal banks -- Zuercher Kantonalbank and Basler Kantonalbank -- are already under investigation by the justice department for possibly having helped Americans evade taxes.

The new participants in the program come as Swiss and U.S. regulators appeal to the institutions to clear up unresolved issues surrounding tax evasion.

Swiss officials have publicly signaled to banks that paying penalties now might be better than risking separate action by the U.S. justice department.

The disclosure program is a key element in an ongoing U.S. effort to get around Switzerland's bank-secrecy laws. Banks won't disclose customers' names, but the U.S. will obtain other data it could potentially use to identify clients. The first deadline for the banks to notify U.S. authorities of their intentions falls at the end of this month.

The program allows banks to register in categories based on the level of undeclared U.S. assets on their books.

The five cantonal banks said they were applying under category 2 of the justice department program. Banks in that category acknowledge they have some U.S. assets on their books and may be liable for fines, but may also receive non prosecution agreements.

All five banks said they had U.S. clients, although those customers had never been the focus of their business.

About a dozen banks already under investigation by the justice department aren't included in the voluntary disclosure program. These include Credit Suisse Group AG CS, -2.73% , Julius Baer Group AG (BAER.VX))and the Swiss branch of HSBC Holdings PLC HSBC, -2.03% . All three banks have said they are cooperating with the probes.

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