The investigation into the two European firms stems from a US tax probe into whether Swiss banking giant UBS helped wealthy Americans dodge taxes through accounts in Switzerland.

The investigation into HSBC and Credit Suisse began around September and is focusing on whether the two banks helped wealthy American clients hide up to US$30 billion (S$46 billion) in offshore accounts that went undeclared to the US Internal Revenue Service, the people briefed on the matter told the newspaper.

HSBC, Credit Suisse and the US Justice Department could not be immediately reached for comment.

Prosecutors are examining whether the two banks illegally helped their American clients use those offshore accounts to evade US taxes and whether the clients themselves violated US laws, the paper said.

The investigations are at an early stage and have not focused on any executives, though that could change as the investigations unfolds, the people told the paper.

The information has emerged, in part, from high-level discussions between senior executives at HSBC and Credit Suisse in the wake of the UBS inquiry, the paper said.

The US authorities are seeking the names of about 17,000 US clients of UBS who have Swiss-based bank accounts.

Swiss lawyers representing US clients of UBS have said Switzerland is considering disclosing information on only a few hundred. -- REUTERS