The Fed also repeated that it plans to keep its key short-term interest rate near zero at least until unemployment falls to 6.5 percent, down from 7.3 percent last month. In the Fed's most recent forecast, unemployment could reach that level as soon as late 2014.
The Fed's short-term rate indirectly affects many consumer and business loans.
"We're in a slow-growth economy with high unemployment and low inflation," said Greg McBride, senior financial analyst at Bankrate.com. "There's no specific catalyst for the Fed to remove stimulus."
The Fed was widely expected to scale back its purchases. But long-term rates on mortgages and some other loans have jumped since May, when Bernanke first said the Fed might slow its bond buys later this year. Bernanke had cautioned that any reduction in purchases would hinge on the economy showing steady improvement.
At his news conference, Bernanke said there's "no fixed schedule" date or "magic number" for when the Fed will slow or end its bond purchases.
In its statement, the Fed said the rise in interest rates "could slow the pace of improvement in the economy and labor market" if they are sustained.
Bernanke also said the Fed is concerned that looming fights between Congress and the White House over the budget and taxes could slow the economy. Unless Congress can agree to fund the government past Oct. 1, a government shutdown will occur.
The government is also expected to reach its borrowing limit next month. Unless Congress agrees to raise the limit, the government won't be able to pay all its bills.
"This is one of the risks we are looking at," Bernanke said.
The Fed also lowered its economic growth forecasts for this year and next year slightly, likely reflecting its concerns about interest rates. It predicts that the economy will grow just 2 percent to 2.3 percent this year, down from its previous forecast in June of 2.3 percent to 2.6 percent growth.
Next year's economic growth will be a barely healthy 3 percent, the Fed predicts.
The Fed's policymakers expect the unemployment rate to fall to between 7.1 percent and 7.3 percent by the end of 2013, slightly below its June forecast of 7.2 percent to 7.3 percent. It predicts that unemployment will fall as low as 6.4 percent next year, down from 6.5 percent in its June forecast.
The Fed's policy statement was approved on a 9-1 vote. Esther George, president of the Federal Reserve Bank of Kansas City, dissented for the sixth time this year. She repeated her concerns that the bond purchases could fuel the risk of inflation and financial instability.
The decision to maintain its stimulus follows reports of sluggish economic growth. Employers slowed hiring this summer, and consumers spent more cautiously.
Super-low rates are credited with helping fuel a housing comeback, support economic growth, drive stocks to record highs and restore the wealth of many Americans. But the average rate on the 30-year mortgage has jumped more than a full percentage point since May and was 4.57 percent last week just below the two-year high.
Investors had bid up those loan rates in anticipation that the Fed would reduce its stimulus as early as this month.
Economists suggested that the Fed will still eventually scale back its bond buying, perhaps before year's end.
"Tapering will come sooner rather than later, assuming that the economy cooperates," Sung Won Sohn, an economist at California State University Channel Islands, wrote in a research report.
The unemployment rate is now 7.3 percent, the lowest since 2008. Yet the rate has dropped in large part because many people have stopped looking for work and are no longer counted as unemployed not because hiring has accelerated. Inflation is running below the Fed's 2 percent target.
