The decision, which takes the Fed into largely uncharted waters, is aimed at further lowering borrowing costs for consumers and businesses still suffering in the aftermath of the worst recession since the Great Depression.

The U.S. central bank said it would buy about $75 billion in longer-term Treasury bonds per month. It said it would regularly review the pace and size of the program and adjust it as needed depending on the path of the recovery.

In its post-meeting statement, the Fed described the economy as "slow", and said employers remained reluctant to add to payrolls. It said measures of inflation were "somewhat low."

"Although the committee anticipates a gradual return to higher levels of research utilization in a context of price stability, progress toward its objectives has been disappointingly slow," the Fed said. (Click here to read Fed statement.)

Stocks showed relatively little reaction to the news. The Dow Jones Industrial Average bounced around between positive and negative, a day after closing at its highest level since April 26. The S&P 500 Index and the Nasdaq also were mostly flat.

Longer-dated U.S. Treasurys shed gains, with 30-year bonds falling more than a point.

The US dollar fell against the euro and also pared gains against the yen.

The central bank repeated its vow to keep the federal funds rate on overnight loans ultra-low for an extended period. Some analysts had speculated the Fed might broaden this commitment.

Kansas City Fed President Thomas Hoenig continued his streak of dissents, saying the risk of additional securities purchases outweighed the benefits.