Less than a day since Angela Merkel threatened Liechtenstein with isolation in Europe unless the principality changed its rules on bank secrecy, German banks have come under the spotlight.

The focus has suddenly shifted to private banks in Germany that may have helped wealthy clients move millions to the tiny Alpine tax haven, sandwiched between Austria and Switzerland.

A few private banks and other financial institutions in Germany helped to administer around 50 anonymous foundations for rich clients in Liechtenstein, Germany's Sueddeutsche Zeitung reported.

The leading state prosecutor in the city of Bochum has publicly confirmed investigators are looking into the role played by some private banks.