His stark language gave a temporary lift to U.S. stocks, but economists walked away from the Fed chairman's remarks still divided over whether the central bank would launch a fresh round of bond purchases at its upcoming meeting in September.

Bernanke said the Fed had to weigh the costs as well as the benefits of more monetary stimulus, although he hinted the costs were likely worthwhile.

"As we assess the benefits and costs of alternative policy approaches ... we must not lose sight of the daunting economic challenges that confront our nation," Bernanke said at the Kansas City Fed's annual Jackson Hole symposium.

"Taking due account of the uncertainties and limits of its policy tools, the Federal Reserve will provide additional policy accommodation as needed to promote a stronger economic recovery and sustained improvement in labor market conditions in a context of price stability."

That was a somewhat weaker hint of policy easing than the minutes of the Fed's last policy meeting had delivered, but Bernanke's dour economic assessment left few doubts where his sympathy lay.

"The stagnation of the labor market in particular is a grave concern not only because of the enormous suffering and waste of human talent it entails, but also because persistently high levels of unemployment will wreak structural damage on our economy that could last for many years," Bernanke said.

Financial markets see-sawed in the wake of Bernanke's comments. U.S. stocks closed roughly where they were before Bernanke spoke, but yields on U.S. government bonds hit a three-week low in anticipation of Fed action and the U.S. dollar fell against both the euro and the yen.

EYES ON THE LABOR MARKET

In response to the financial crisis and recession of 2007-09, the Fed cut overnight interest rates to near zero and bought $2.3 trillion in government and mortgage securities in two separate rounds of so-called quantitative easing.

It next meets on September 12-13, and policymakers have been locked in debate over whether further bond purchases are warranted to spur a stronger recovery.

Economists said Bernanke's emphasis on the health of the job market throws an especially strong spotlight on a report due on September 7 on job growth in August. Hiring picked up in July but the jobless rate moved up to 8.3 percent.

"The speech did not add anything to the information that we had, but, importantly, it did not subtract anything either," said Roberto Perli, managing director of policy research at the International Strategy and Investment Group in Washington.

"It did not walk back an inch from the fairly dovish tone of the minutes," he said, adding it was "probably more a question of when, not if" on further asset purchases.

DOWNPLAYS RISKS OF UNCONVENTIONAL POLICIES

The Fed's aggressive efforts to prop up the economy have drawn criticism from Republican politicians for potentially sowing the seeds for inflation and asset bubbles.

Republican presidential hopeful Mitt Romney has said he does not think a third round of quantitative easing, or QE3 in market parlance, would help the economy, and some analysts think the central bank may be hesitant to act ahead of the November 6 presidential election. After September, the Fed has one more policy meeting in late October before Americans go to the polls.

Bernanke, however, downplayed the potential risks from the Fed's unconventional policies and argued the asset purchases had been quite effective at boosting economic growth.

"The costs of nontraditional policies, when considered carefully, appear manageable," he said.

The economy emerged from recession nearly three years ago, but growth has remained tepid. U.S. gross domestic product advanced at a 1.7 percent annual rate in the second quarter, too weak to bring down the nation's elevated unemployment rate.

"Unless the economy begins to grow more quickly than it has recently, the unemployment rate is likely to remain far above levels consistent with maximum sustainable employment," Bernanke said. The Fed is charged with pursuing both price stability and full employment.

Economic data has improved since the Fed's July 31-August 1 meeting, which came before the stronger-than-expected reading for July employment. Reports on retail sales, exports and housing have also been relatively solid.

A report on Friday showed U.S. consumer sentiment hit a three-month high in August, although pessimism on the future remained.

The economy's generally better tone has led some market participants to dial back their expectations of a fresh round of Fed bond purchases in September.

As an alternative, many economists say, the Fed may simply push further into the future the date it thinks it will finally start to move interest rates higher. The central bank has said since January that it expects to keep rates near zero at least through late 2014.

(Writing by Pedro da Costa and Tim Ahmann; Editing by Tim Ahmann, Andrea Ricci and Chizu Nomiyama)

• A Permanent Economic Emergency