The Fed said it would buy the long-term government bonds by the middle of 2011 to further drive down interest rates on mortgages and other debt.
This is in addition to an expected $US250 billion ($A250.8 billion) to $US300 billion ($A300.96 billion) in Fed purchases over the same period from reinvesting proceeds from its mortgage portfolio.
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The idea is for cheaper loans to get people to spend more and stimulate hiring. The Fed said it will monitor whether adjustments are needed depending on how the economy is performing.
Some worry the Fed action will do little to boost the economy because interest rates are already historically low. Others fear the bond purchases could drive inflation too high over the long term and unleash speculative buying in assets like stocks.
In announcing the action, the Fed said the pace of the economy continues to be slow. Companies remain reluctant to hire, housing activity is depressed and consumers are increasing their spending only gradually.
Thomas Hoenig, president of the Federal Reserve Bank of Kansas City, dissented for the sixth straight meeting. He says the risks of the Fed's extra stimulus outweigh the potential benefits.
With the economy weak, the Fed doesn't want to take chances that it could get stuck in the kind of economic stagnation and deflation problems that gripped Japan and led to a "lost decade" during the 1990s.
Deflation is a widespread and prolonged drop in prices of goods and services, in wages and in the values of homes and stocks.
Deflation makes it harder for people and companies to pay their debts, pushing up home evictions and bankruptcies.
Fed policymakers expressed disappointment that they haven't been able to reduce unemployment - now at 9.6 - and raise inflation to levels more in line with a healthy economy. Progress toward those goals has been disappointingly slow, the Fed acknowledged in its post-meeting statement.
The Fed has tried since the 2008 financial crisis to keep credit available to individuals and businesses. It's done so, in part, by keeping the target range for its bank lending rate near zero.
It also pursued the unorthodox strategy of buying long-term bonds. The Fed's purchases are so vast that they push down the rates on those bonds.
In 2009, the Fed bought $US1.7 trillion ($A1.71 trillion) in mortgage and Treasury bonds. Those purchases helped lower long-term rates on home and corporate loans. The program was credited with helping to lift the country out of recession.
The action comes one day after voters frustrated by unemployment, scant pay gains and soaring home foreclosures
punished Democrats and handed control of the House to Republicans.
Democrats kept control of the Senate.
The split will make it harder for President Barack Obama to enact any major economic initiatives. That could put more pressure on the Fed to get the economy back on firmer footing.
Once again, the Fed pledged to hold its key interest rate at a record low near zero for "an extended period."
