Moving in a new way to boost the U.S. economy, the Federal Reserve said Wednesday it will buy up to $300 billion US of long-term government securities over the next six months.

Saying the U.S. economic situation has deteriorated since its January rate decision, the U.S. central bank said it will also buy up to $750 billion US in additional mortgage-backed securities, bringing its total purchases of these securities up to $1.25 trillion US this year.

The move came as the Fed left its key interest rate unchanged at a range between zero and 0.25 per cent. The central bank said it expects interest rates will remain low for "an extended period" to bolster the U.S. economy, which has been in recession since December 2007.

"Job losses, declining equity and housing wealth, and tight credit conditions have weighed on consumer sentiment and spending," the Fed said.

"Weaker sales prospects and difficulties in obtaining credit have led businesses to cut back on inventories and fixed investment. U.S. exports have slumped as a number of major trading partners have also fallen into recession."

Buying up U.S. government securities is a new move for the Fed as it tries to stimulate the economy. Purchasing the Treasury debt would bolster their prices and reduce their yields, which would have the effect of making borrowing cheaper on debt, such as mortgages, tied to Treasury rates.