Bernanke sought to lower expectations about how much the Fed’s intervention might help the economy.
‘‘We’re just trying to get the economy moving in the right direction, to make sure that we don’t stagnate at high levels of unemployment,’’ he said. ‘‘All that being said, monetary policy, as I've said many times, is not a panacea.’’
The Fed’s statement was approved 11-1. The lone dissenter was Richmond Fed President Jeffrey Lacker, who worried about igniting inflation.
The Fed’s new bond purchases, which will start Friday, amount to less per month than either of its first two bond programs. But by committing to buying bonds indefinitely, the Fed is seeking to assure investors and consumers that borrowing will remain cheap far into the future.
‘‘In many ways, today’s actions represent the beginning of a new phase in Bernanke’s efforts to get the economy moving again,’’ said Michael Feroli, an economist at JPMorgan Chase Bank.
Some economists suggested that the Fed might continue to buy $40 billion a month in mortgage bonds for up to three years. That’s how long some expect it will take for the unemployment rate to dip below 7 percent, toward a ‘‘normal’’ rate of 6 percent or less.
If the new bond buying lasts three years, Ashworth said it would add about $1.4 trillion to the Fed’s purchases. That would be close to the $1.7 trillion the Fed spent in its first round of bond buying, which began in November 2008 and ran until March 2010.
The Fed’s second bond-buying program totaled $600 billion. It ran from November 2010 through June 2011.
Still, skeptics caution that further bond buying might provide little economic benefit because rates are already near record lows. Critics also warn that more bond purchases raise the risk of higher inflation later.
A spokeswoman for Mitt Romney’s Republican presidential campaign said the Fed’s latest efforts to boost the economy are ‘‘further confirmation that President Obama’s policies have not worked.’’
The Fed is under pressure to act because the U.S. economy is still growing too slowly to reduce high unemployment. The unemployment rate has topped 8 percent every month since the recession officially ended more than three years ago.
In August, job growth slowed sharply. Employers added just 96,000 jobs, down from 141,000 in July and well below what is needed to bring relief to the more than 12 million who are unemployed.
The unemployment rate did fall to 8.1 percent from 8.3 percent. But that was because many Americans stopped looking for work, so they were no longer counted as unemployed.
Bernanke spotlighted the problem of chronic high unemployment in a speech to an economic conference in Jackson Hole, Wyo., late last month. He argued that bond purchases and other unorthodox Fed actions had helped ease borrowing costs and boost stock prices.