She also called for greater transparency in financial markets and hedge funds in a speech during a visit to Japan, echoing German politicians who have demanded tighter regulation of the banking sector in the wake of two major industry bail-outs.
"There is also little transparency at the ratings agencies that rate certain companies. If we look at how the mortgage crisis happened, we have often experienced that what was rated highly in the end turned out to be much more unstable," she said.
German banks have been the hardest hit in Europe by troubles in the risky US subprime mortgage sector.
On Sunday, German bank SachsenLB was bought by a state-backed bank in a hasty rescue deal after nearly collapsing due to its exposure to the subprime mortgage sector.
Small-company lender IKB also needed an industry bail-out after its subprime-related investments crumbled.
Rating agencies Moody's Investors Service and Standard & Poor's have warned that banks may see double-digit falls in revenues and pretax profits due to credit and mortgage market troubles. "It's not acceptable that wrong risk assessment in one place has to be paid for by the entire global community," Merkel told a symposium in Tokyo organised by the Nikkei business daily.