WASHINGTON (Reuters) - Interest rates on 30-year U.S. mortgages fell below 4 percent for the first time this week, helping those with access to credit but doing little for the millions who owe more than their homes are worth.

Freddie Mac, a major mortgage finance provider controlled by the U.S. government, said on Thursday the national average rate for 30-year fixed rate mortgages dropped to 3.94 percent in the week through October 6.

That is the lowest level on record dating to 1949 for mortgages with terms of 25 years or more. Last week, 30-year rates averaged 4.01 percent.

The U.S. Federal Reserve slashed overnight interest rates to near zero in December 2008 to try to lower costs for mortgages and other loans, and it has continued to break out new tools to help the economy recover from recession.

After its last meeting on September 21, the central bank said it would alter the composition of its bond portfolio in an attempt to further lower borrowing costs, and said it would continue its support for mortgage-related debt.

But many economists are skeptical those attempts to lower rates will help much because millions of Americans owe more on their mortgages than their homes are worth. That can effectively chain them to their properties while also preventing them from refinancing to lower their monthly costs.

Household debt taken on during the pre-recession boom years is holding back the economy's recovery.

Freddie Mac's records on 30-year rates go back to 1971, but data on long-term mortgages from the National Bureau of Economic Research show rates never dropped below 4 percent.

NBER data on 30-year mortgages extends back as far as 1961 and data on 25-year mortgages to 1949.

(Reporting by Jason Lange; Editing by Kenneth Barry)