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. Fed officials also have indicated a desire to insure against looming risks to the recovery, including problems in Europe and the stalemate in domestic fiscal policy.


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 “growth in employment has slowed in recent months,” even as inflation has declined.


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, maintaining its portfolio at roughly the same size.


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 fourth such program since 2008.


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 action this year.


The statement concluded with the Fed’s standard affirmation under the leadership of the Fed chairman, Ben S. Bernanke, that it remains “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.”


The decision to extend Operation Twist was widely expected by financial analysts, who viewed it as something of a placeholder, a way of maintaining the status quo while the Fed sought greater clarity about the long-term health of the economy.


Studies of the first installment of Operation Twist, which began last fall, have concluded that it reduced interest rates by around 0.15 to 0.20 percentage points. But its impact, like those of earlier purchase programs, has been muted by the inability of many businesses and consumers to obtain loans.


The Fed has reduced borrowing costs for businesses and consumers through a range of measures. It has kept short-term interest rates near zero since late 2008, and said that it planned to maintain that policy until at least late 2014. It has bought more than $2.5 trillion in Treasuries and mortgage securities to further reduce long-term interest rates. And in the program that was scheduled to end this month, it shifted the composition of its portfolio to bear down even harder on long-term rates.


But a growing collection of lukewarm economic data has appeared to crack the united front Fed officials presented earlier this year, when they proclaimed the belief — not for the first time — that the central bank finally had done enough to revive growth.


Eric S. Rosengren, president of the Federal Reserve Bank of Boston, has issued public calls in recent weeks for the Fed to expand its efforts, and others who had argued for more aggressive easing last year also seemed to be girding recently 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 the vast loss of wealth and jobs 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 further.


After falling rapidly in the closing months of 2011, the unemployment rate has stalled above 8 percent during the first half of this year. But the Fed’s growth projections released in April suggested that the economy would expand fast enough to modestly reduce the rate during the rest of the year.


Mr. Bernanke told Congress in testimony earlier this month that the Fed would consider additional action if it concluded that the economy would not grow fast enough to reduce unemployment, as inflation was under control.


The decision is further complicated by a pair of looming risks: that events in Europe will freeze global financial markets and that political dysfunction in Washington will undermine the domestic recovery. Some influential members of the Fed’s policy committee argued in recent weeks that the Fed should consider increasing its efforts to stimulate the economy as an insurance policy against potential disruptions.