The US Federal Reserve is to buy up to $US300 billion worth of longer-term US government debt over the next six months.
It will also expand purchases of mortgage-related debt to help ease credit market conditions.
In a statement on Wednesday at the end of a two-day meeting, the fed's policy panel also said its target for overnight interest rates will be held in a zero to 0.25% range.
It repeated that borrowing costs would likely stay unusually low for some time.
