WASHINGTON – The Federal Reserve announced plans Wednesday to inject hundreds of billions of dollars into the U.S. financial system, in an expansive and unconventional new effort to try to get the sputtering U.S. economy on track.

The Fed will, in effect, print money to buy Treasury bonds — an extra $600 billion worth by June 2011 — in a bid to lower long-term interest rates. The action should make it cheaper for Americans to borrow money, take out a mortgage or refinance their house, and for businesses to borrow funds in order to expand.

Although widely anticipated, the move was bolder than analysts had predicted. Still, some investors had expected the Fed might announce an even more aggressive package of bond purchases, and interest rates in financial markets rose Wednesday after the announcement.

The yield on 30-year Treasury bonds jumped 0.17 percentage points to 4.04 percent as of 5 p.m. The rate on 10-year Treasury bonds initially rose 0.04 percentage points to 2.6 percent, but later slipped back to settle at 2.57 percent.

The stock market advanced after the announcement. The Standard & Poor’s 500-stock index rose 0.4 percent to close at 1197.96.

In a statement accompanying the decision, the Fed’s policymaking committee emphasized that the action — a step known as “quantitative easing” — was driven by stubbornly high unemployment and ultra-low inflation.

Information on the economy received by Fed members since the last policy committee meeting in September “confirms that the pace of recovery in output and employment continues to be slow,” the Federal Open Market Committee statement said. “To promote a stronger pace of economic recovery and to help ensure that inflation, over time, is at levels consistent with its mandate,” the Fed will buy about $75 billion in Treasury bonds a month over the coming eight months.

A related announcement by the Federal Reserve Bank of New York noted that, combined with Treasury bond purchases already announced to replace maturing mortgage securities on the central bank’s balance sheet, the Fed will be buying $850 billion to $900 billion in bonds within that time span.

The statement also made clear that the Fed will keep its options open, potentially extending the purchases if the economy continues to underperform, or even reducing them if growth were to spike.

If the Fed’s latest strategy works as planned, it will help strengthen an economy that has been stuck in a pattern of growth too slow to bring down joblessness. Yet even many advocates of the approach stress that it is no cure-all. Its ultimate impact on the economy depends on whether consumers and businesses respond to lower interest rates by buying and investing. If they continue sitting on cash, the effect could be minimal.

Moreover, the Fed’s new path has risks. Inflation could spike down the road, creating bubbles in the stock market or housing prices, or causing the dollar to decline rapidly. For these reasons, several top Fed officials have expressed resistance to the move, including Thomas Hoenig of the Kansas City Fed.

Wednesday’s action is a recognition by the central bank that it is falling short on both of the mandates with which it is legally charged — maintaining maximum employment (the national jobless rate was 9.6 percent in September) and keeping prices stable (Fed officials view 2 percent inflation as best for long-term price stability, not the 1 percent of late).

Indeed, Fed officials, including Chairman Ben Bernanke, have explicitly emphasized that pushing inflation a bit higher is one of the goals behind the new action. Bernanke also has stressed that monetary policy cannot be viewed as a sole solution to the nation’s economic woes.