The US Federal Reserve has moved to counter what it said are significant risks to the American economy with an effort to lower long-term borrowing costs and bolster housing.
The US central bank said it would launch a $US400 billion programme to weight its $US2.85 trillion balance sheet more heavily toward longer-term securities by selling short-term government debt to purchase longer-dated Treasuries.
It will also reinvest proceeds from maturing mortgage and housing agency bonds it holds back into the mortgage market - an acknowledgment of just how weak housing remains.
The US economy grew at less than an annual rate of 1% over the first half of the year and economists have warned of a heightened risk of recession.
Analysts have questioned whether the central bank's move will have much impact.