To spur borrowing and spending, it's extending a program designed to lower long-term U.S. interest rates.
At the end of a two-day policy meeting Wednesday, the Fed also sharply reduced its forecast for U.S. growth and said it's prepared to act further to bolster the economy. It reiterated its plan to keep short-term interest rates at record lows until at least late 2014.
"If we're not seeing a sustained improvement in the labor market, that would require additional action," Bernanke said at his quarterly news conference later in the day.
Wall Street wasn't impressed by the Fed's limited response Wednesday. Stock prices barely budged. And analysts questioned how much benefit the Fed's latest economy-boosting effort would have, in part because interest rates are already near record lows.
If the Fed's more pessimistic outlook proves accurate, President Barack Obama's chances in an election that will turn on the economy would likely suffer.
Bernanke noted that the economy is under threat from Europe's debt crisis and the prospect of sharp spending cuts and tax increases that would take effect at the end of the year without a congressional agreement.
European leaders will be seeking a breakthrough at a summit next week in Brussels. Bernanke said he's in regular touch with the head of the European Central Bank.
The Fed said in a statement around 12:30 p.m. EDT that it will continue a program called Operation Twist through year's end. Under the program, the Fed has been selling $400 billion in short-term Treasurys since September and buying longer-term Treasurys. It said it will extend the program through December using $267 billion in securities.
But extending Operation Twist might not provide much benefit. Businesses and consumers who aren't borrowing now might not do so if rates slipped slightly more.
"This move is largely symbolic," said David Jones, chief economist at DMJ Advisors.
Jones estimates Operation Twist will lower long-term rates by only about one-tenth of a percentage point.
At his quarterly news conference later Wednesday, Bernanke said the Fed would consider more aggressive action, such as another bond buying program. The Fed has completed two such programs. It bought more than $2 trillion in Treasurys and mortgage-backed securities, expanding its portfolio above $2.8 trillion.
The yields on Treasury bonds finished the day only slightly below where they were before the announcement. The Dow Jones industrial average finished down about 13 points.
John Canally, investment strategist at LPL Financial, says the Fed delivered just what investors expected and offered a hint at further easing.
"If there's another misstep somewhere - in Europe ... more weak data - the Fed's going to do more," Canally said.
For now, he said, the Fed wants to keep "some powder dry" in case there's a meltdown in Europe. Canally also suggested that the Fed may be reluctant to be aggressive in an election year out of concern it could be seen as affecting the election.
But in a comment on Twitter, Justin Wolfers, an economics professor at the University of Pennsylvania's Wharton Business School, suggested that the Fed might be on the cusp of going further.
Wolfers characterized their view as: "One more bad jobs report and we'll do more."
The Fed now thinks the economy will grow no more than 2.4 percent this year. That compares with its forecast in April that the economy could grow up to 2.9 percent.
And it thinks the unemployment rate, now 8.2 percent, won't fall much further in 2012.In its statement, the Fed noted that oil and gas prices have fallen. Lower prices give the Fed room to take further action without igniting inflation.
The Fed's statement was approved on a 11-1 vote. Jeffery Lacker, president of the Richmond Regional Fed Bank, dissented for the fourth straight meeting. The statement said he opposed the continuation of Operation Twist.
Josh Feinman, global chief economist at DB Advisors, said the extension of Operation Twist allows the Fed to do something without expanding its portfolio of securities. Launching a new bond-buying program would have likely incited criticism that the Fed was escalating the long-term risks to the economy.
In part, that's because Fed would eventually find it harder to shrink its portfolio without driving interest rates back up and possibly threatening the economy.
"The downside risks have increased enough that they felt they needed to do something," Feinman said. "Extending Operation Twist was the path of least resistance."
Job growth averaged only 73,000 in April and May, after average gains of 226,000 a month in the first three months of the year.
The number of people seeking unemployment benefits has risen about 5 percent in the past six weeks. And employers posted sharply fewer job openings in April compared to the previous month.
Economists also worry the debt crisis in Europe is worsening, even after Greek election results increased the likelihood that Greece will stay in the euro currency alliance.
One positive trend is that U.S. inflation is low. Core consumer prices, which exclude volatile food and energy costs, have risen just 2.3 percent over the past 12 months. That's near the Fed's 2 percent target for inflation.
Critics have complained about the Fed's efforts to boost growth over the past three years by purchasing more than $2 trillion in bonds. They say the extra money added to the banking system could ignite inflation once the economy rebounds.
This week's Fed meeting was the first time that the Fed board has been at full strength in six years. Jeremy Stein, a Harvard economics professor, and Jerome Powell, a former private equity executive, attended their first policy meeting since being confirmed by the Senate last month.