WASHINGTON — The Federal Reserve announced Wednesday a modest increase in its efforts to reduce borrowing costs for businesses and consumers by extending its existing “Operation Twist” asset-purchase program through the end of the year.

The decision reflects growing concern that the economy once again is stumbling into the summer months after the false promise of a relatively strong winter. The Fed now expects the unemployment rate to fall no lower than 8 percent this year, and inflation to rise no higher than 1.7 percent, both signs of an ailing economy.
Fed officials also have indicated a desire to insure against a pair of looming risks, that events in Europe will freeze global financial markets and that the political stalemate in Washington over fiscal policy will undermine the domestic recovery.
But the program extension does not appear large enough to boost growth significantly. Instead, it amounts to a placeholder, an effort to soothe markets and preserve the status quo while the Fed seeks greater clarity about the health of the economy, economists said.
The Fed’s chairman, Ben S. Bernanke, said the situation “is not entirely clear.”
“We have to get further information about the state of the economy, about where things are going and about what’s happening in Europe,” Mr. Bernanke said at a news conference following the release of the policy statement and projections.
“We are prepared to do what is necessary,” he said, repeating a promise that has become his byword. “We are prepared to provide support for the economy.”
Fed officials said they now expected the economy to expand between 1.9 percent and 2.4 percent this year, down from an April forecast of 2.4 percent to 2.9 percent.
The economic forecast, released separately, reflected reduced prospects for 2013 as well. The Fed estimated growth of between 2.2 percent and 2.8 percent, down from 2.7 percent and 3.1 percent in the April forecast.
Growth at that pace would barely dent unemployment and, indeed, the Fed also reeled in its expectations for a continued decline in the unemployment rate. It now expects the rate to sit between 7.5 and 8 percent at the end of 2013, up from an April forecast of 7.3 to 7.7 percent.
The Fed’s policy-making committee said in a statement that it expected the economy would continue to grow at a “moderate pace,” but it noted that employment growth and household spending both had slowed in recent months. Mr. Bernanke said that the economic downturn in Europe was taking a clear toll on the American economy.
In response, the Fed said that it would buy about $267 billion in longer-term Treasury securities over the next six months, with money raised by selling some of its current holdings of short-term Treasuries. The Fed already has purchased $400 million in long-term securities at the same monthly pace since last September.
Studies of this first installment of Operation Twist have concluded that it reduced interest rates by around 0.15 to 0.20 percentage points. But its impact, like those of the Fed’s earlier purchase programs, has been muted by the inability of many businesses and consumers to obtain loans. Economists estimate that this second installment will have an even smaller impact, commensurate with its size.
“It will do very little to boost growth, but the Fed clearly wants to be seen to be doing something,” Ian Shepherdson, chief United States economist at High Frequency Economics, wrote in a note to clients shortly after the decision was announced.
It is the first time since January that the Fed has intensified its efforts to revive economic growth, and the first time since September that the Fed has announced a new round of asset purchases. This is the fifth such announcement since 2008. In total, the Fed has purchased well more than $3 trillion in securities.
The decision was supported by 11 members of the committee. The only opposition came from Jeffrey M. Lacker, president of the Federal Reserve Bank of Richmond, who has repeatedly dissented from the Fed’s actions this year.
The statement concluded with the standard affirmation that the Fed is “prepared to take further action as appropriate to promote a stronger economic recovery and sustained improvement in labor market conditions in a context of price stability.”
Republicans have been particularly critical of the Fed’s efforts to revive growth, warning that the central bank has done all that it can and that additional efforts are more like to spark higher inflation than lower unemployment.
Mr. Bernanke was dismissive of those criticisms.
“I wouldn’t accept the proposition that the Fed has no more ammunition,” he said.
The committee did not change its prediction that the Fed would hold interest rates near zero until late 2014, at least, but support for that policy was somewhat stronger than at the April meeting, with members of the committee generally forecasting a slower increase in interest rates.
In addition to Operation Twist, the Fed has reduced borrowing costs for businesses and consumers by keeping short-term interest rates near zero since late 2008, and by purchasing more than $2.5 trillion in Treasuries and mortgage securities to further reduce long-term interest rates. Earlier this year, Fed officials appeared united in the view that those policies would be sufficient.
But a growing pile of lukewarm economic data cracked that united front.
Eric Rosengren, president of the Federal Reserve Bank of Boston, issued a public call for the Fed to expand its efforts, and others who had argued for more aggressive easing last year also seemed recently to be girding for a return to the barricades. The Fed’s conservative wing, meanwhile, renewed its warnings about future inflation.
The moderate group that has controlled the course of policy, led by Mr. Bernanke, has wavered in the middle. The economy appears to be growing at a moderate pace that would attract little comment in normal times, but that is far too slow to make up for vast losses of wealth and jobs sustained during the recession. Mr. Bernanke and his allies have shown little desire to drive growth much above this modest pace, but several reiterated in recent weeks that they would be inclined to respond if economic activity slipped any further.
Wednesday’s announcement will do little to ease that pressure, particularly if forthcoming economic data continues to disappoint investor expectations.
“I read the Fed as saying: ‘One more bad jobs report, and we’ll do more,’ ” Justin Wolfers, an economist at the University of Pennsylvania, wrote in a Twitter message.