The number of foreign banks that are now pursuing multi-million to billion-dollar residential mortgage-backed securities cases against U.S.-based financial firms over toxic mortgages is quietly growing.

In fact, the foreign investor equation in the RMBS fallout seems to be hidden even if the cases are actually quite public. And like other RMBS disputes, they keep flowing in.

Case in point, Law360 reports that German bank DZ Bank AG is suing Morgan Stanley [stock MS] [/stock] over close to $700 million in RMBS sold to DZ Bank. The case is like numerous others with accusations of misrepresentations made about the quality of the underlying mortgages.

DZ Bank is not the first German bank to go after major investment banks on the RMBS issue. In September, Bayerische Landesbank [stock BLGZ] [/stock], a subsidiary of BavernLB, slapped Goldman Sachs [stock GS] [/stock] with a lawsuit, claiming it acquired $511.9 million in residential mortgage-backed securities from Goldman on faulty underwriting information.

And last January, Belgium bank Dexia and Bayerische Landesbank sued Morgan Stanley over $1.2 billion in RMBS.

Furthermore, Irish investment vehicle Sealink Funding is suing Citigroup [stock C] [/stock] over $513.3 million in residential-mortgage backed securities deals that allegedly contained toxic underlying mortgages.

Sealink also has filed suit against JPMorgan [stock JPM] [/stock], Bank of America [stock BAC] [/stock] and Royal Bank of Scotland [stock RBS] [/stock] for the firms' involvement in the securitization and sales of mortgage bonds backed by what were alleged to be poorly underwritten loans.

The flow of cases from foreign plaintiffs comes at a time when analysts are still trying to figure out the various risks that banks face when it comes to RMBS litigation and putback exposure.