Fannie Mae and Freddie Mac may have lost more than $3 billion as a result of banks' alleged manipulation of a key interest rate, according to an internal report by a federal watchdog sent to the mortgage companies' regulator and reviewed by The Wall Street Journal.

The unpublished report urges Fannie and Freddie to consider suing the banks involved in setting the London interbank offered rate, which would add to the mounting legal headaches financial firms such as UBS and Barclays face from cities, insurers, investors and lenders over claims tied to the benchmark rate.

The findings are not conclusive, the WSJ adds.