Reducing the Fed's efforts to keep borrowing rates low would "carry a substantial risk of slowing or ending the economic recovery," Bernanke said in testimony to the Joint Economic Committee, a panel that includes members of the House and Senate.
The Fed has been buying $85 billion a month in Treasury and mortgage bonds since September. That has helped lower long-term interest rates and encouraged more borrowing and spending.
Lawmakers pressed Bernanke to explain when the Fed might start to scale back its purchases. Bernanke said the pace could be reduced over the next few meetings, if the job market shows "real and sustainable progress." And he wouldn't rule out curtailing the purchases by Labor Day.
But Bernanke said that the Fed could just as quickly reverse course and pick up the pace if the economy falters.
Most of his testimony focused on the many risks facing the economy, along with the benefits gained so far from the Fed's stimulus. His comments suggest the Fed is not ready to taper the bond purchases.
Stocks surged after Bernanke spoke. The Dow Jones industrial average gained 94 points in midday trading. It was up just 40 points before the hearing began at 10 a.m. EDT.
Paul Ashworth, an economist at Capital Economics, said Bernanke's remarks suggest "he is in no hurry to curb" the bond purchases. Ashworth predicts the central bank will begin to trim the bond purchases toward the end of the year and end them completely in the first half of 2014.
At the hearing, Bernanke declined to answer a direct question about whether he would consider serving another four-year term when his current term ends in January.
The Fed has said it plans to continue its $85 billion-a-month in Treasury and mortgage bond purchases until the job market improves substantially. And after its April 30-May 1 meeting, the Fed said it could increase or decrease the pace depending on how the job market and inflation fare.
Bernanke noted that the economy is growing moderately this year and unemployment has fallen to a four-year low of 7.5 per cent. Still, unemployment remains well above levels consistent with healthy economies. And Bernanke said higher taxes and deep federal spending cuts are expected to slow economic growth this year.
In recent months, the job market and the broader economy have shown renewed vigour. The economy has added an average of 208,000 jobs a month since November. That's up from only 138,000 a month in the previous six months.
The economy has benefited from a resurgent housing market, rising consumer confidence and the Fed's stimulus actions, which have helped ignite a stock market rally. The Standard & Poor's 500 stock index has jumped 17 per cent this year to a record high. Higher stock prices tend to make many people feel wealthier and more inclined to spend.
Those gains, in part, are why critics of the bond purchases, including some Fed regional bank presidents, have questioned the need to continue them at their current pace. They argue that keeping interest rates too low for too long could send inflation surging or inflate dangerous bubbles in assets such as stocks or real estate. Such a bubble could burst with the same destabilizing effects that the housing bust caused.
The panel's chairman, Rep. Kevin Brady, R-Texas, is among those critics. On Wednesday, he pressed Bernanke to explain when the Fed might begin to reduce its bond purchases.
Bernanke said the Fed could take a "step down" if the job market shows "real and sustainable progress" over the next few meetings.
When asked if that could happen before Labor Day, Bernanke responded: "I don't know. It will depend on the data."
But Bernanke made clear that the Fed could also increase its purchases if the economy began to weaken. "We could raise or lower our purchases going forward," Bernanke said.
Bernanke has had solid support for the bond purchases among the voting members of the Fed's interest-rate setting committee. At each of the Fed's three policy meetings this year, the committee has approved the purchases 11-1.