Standard & Poor's has downgraded mortgage giants Fannie Mae and Freddie Mac from triple-A to AA+, citing their direct reliance on the US government, which it downgraded on Friday last week.
S&P also downgraded 10 out of 12 Federal Home Loan Banks and the senior debt issued by the Federal Farm Credit Banks on Monday.
The downgrades were expected after S&P cut its US sovereign rating also from AAA to AA+, citing a dangerously rising debt burden and the inability of Washington's battling politicians to come up with a credible long-term policy to cut the country's deficits.
Fannie and Freddie are businesses that buy up mortgages from banks and mortgage brokers to keep rates low and help home buyers. They now hold about half of all US mortgages.
Always under government oversight, both had to be rescued after the 2007-2008 financial crisis.
"The downgrades of Fannie Mae and Freddie Mac reflect their direct reliance on the US government. Fannie Mae and Freddie Mac were placed into conservatorship in September 2008 and their ability to fund operations relies heavily on the US government," S&P said.
"In addition to the implicit support we factor into our ratings, the US Treasury has demonstrated explicit support by providing these entities with capital quarterly, as necessary."
Meanwhile, S&P said the US sovereign downgrade did not affect its ratings of money market funds investing in US debt, especially its short-term debt.
But it warned that a decline in the prices of Treasuries and government securities could hit the net asset values of these money market funds.