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.

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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.

David Fuller's view The US Federal Reserve and just about every other central bank will remain accommodative, through 2H 2012 and possibly well beyond. This remains a tailwind for equities, capable of at least partially offsetting the headwind from concerns over slower global GDP growth.