"Participants at the June FOMC (Federal Open Market Committee) meeting indicated that they see a higher degree of uncertainty about their forecasts than normal and that the risks to economic growth have increased," Bernanke said in his semi-annual monetary policy testimony before the Senate Banking Committee.

The U.S. economy has continued to recover, but economic activity appeared to have decelerated somewhat during the first half of this year, said the central bank chief.

After rising at an annual rate of about 2.5 percent in the second half of 2011, U.S. real gross domestic product (GDP) expanded at a nearly 2 percent pace in the first quarter of this year, and "available indicators point to a still-smaller gain in the second quarter" in 2012, said Bernanke in his testimony.

Bernanke highlighted the renewed intensification of euro area crisis and the U.S. fiscal uncertainty as the two main sources of risks to the U.S. economic recovery.

Although eurozone authorities have announced a number of measures to cope with eurozone debt crisis, including funding for the recapitalization of Spain's troubled banks, Europe's financial markets and economy remained under "significant stress," with spillover effects on financial and economic conditions in the rest of the world, including the United States, he added.

The possibility that the situation in Europe might worsen further remained a significant risk to the U.S. outlook, said Bernanke, urging U.S. Congress to address the nation's fiscal challenges in a way that takes into account both the need for long- run sustainability and the fragility of the economic recovery.