The divide underscores the hurdles Fed Chairman Ben Bernanke faces as he tries to win consensus among his fellow policymakers on the central bank's sometimes controversial efforts to bring down the nation's lofty unemployment rate, which registered 7.9 percent last month.

Charles Evans, president of the Chicago Federal Reserve Bank and one of the Fed's most outspoken doves, said interest rates should stay near zero until the jobless rate falls to at least 6.5 percent. Such a policy would carry "only minimal inflation risks," and could boost growth faster than otherwise, he said.

Evans, who rotates into a voting seat on the Fed's policy-setting panel in January, also said the Fed should step up its program of quantitative easing in the new year to keep its overall level of asset purchases at $85 billion a month for most, if not all, of 2013.

But Dallas Fed President Richard Fisher, a self-identified inflation hawk, said the U.S. central bank could get into trouble if it does not set a limit on the amount of assets it is willing to buy.

"You cannot expand without limits without horrific consequences," he told reporters on the sidelines of the conference organized by the Levy Economics Institute in Berlin. "There is no infinity in monetary policy, we know that from the German experience."

In September the Fed launched an open-ended asset-purchase program, kicking it off with a monthly $40 billion in mortgage-backed securities and promising to continue or ramp up the program unless the outlook for the labor market improves substantially.

Those purchases come on top of the $45 billion in long-term Treasuries the Fed is buying each month under Operation Twist, purchases that are funded with sales of a like amount of short-term Treasuries.

"It's important to maintain the overall level of asset purchases at $85 billion, at least for a time until we can see whether or not we are doing better or things are going more slowly, and we can adjust, depending on that assessment," Evans told reporters attending a speech at the C.D. Howe Institute in Toronto

"I think we have to have discussion about what is 'substantial improvement.' Have we seen it? In my opinion, we have not," he said.

Evans said he would judge the labor market as substantially improved once he sees monthly job gains of a least 200,000 for about six months, as well as above-trend growth in gross domestic product that would lead to declines in unemployment.

"I would be very surprised if we could achieve that before six months have passed, and I would not be surprised if it takes until the end of 2013," he said.

Evans said the Fed should keep rates low well beyond that date, until the jobless rate hits at least 6.5 percent, as long as the inflation outlook for the next two to three years remains below 2.5 percent. The Fed's inflation target is 2 percent.

Evans for the past year had called for low rates until the jobless rate falls to 7 percent, as long as inflation does not threaten to breach 3 percent.

On Tuesday Evans said he now views a 7 percent unemployment threshold as "too conservative," and sees a 2.5 percent inflation safeguard as appropriate, given that a higher threshold makes some people "apoplectic" and is not needed in order for the policy to work.

"We're much more likely to reach the 6.5 percent unemployment threshold before inflation begins to approach even a modest number like 2.5 percent," he said.

Fed policymakers have been ramping up discussions on so-called thresholds - economic data points such as specific unemployment and inflation rates - that would signal when the central bank is likely to begin raising benchmark interest rates from near zero.

Minneapolis Fed President Narayana Kocherlakota, Boston Fed President Eric Rosengren and the Fed's influential vice chair, Janet Yellen, have all expressed support for the idea.

In Berlin, Fisher also chimed into the debate.

"One option I believe we might pursue is to have a definition of our unemployment target as well as our long-term inflation target," he said, noting it would be difficult however and setting an overall limit on asset purchases was preferable.

Fed Chairman Bernanke said last week that adopting numerical thresholds for unemployment and inflation could be a "very promising" step to develop the Fed's communication strategy, but stressed that it was still under discussion.

On at least one issue, Fisher and Evans agreed: lack of jobs, not high inflation, is the biggest problem for the U.S. economy.

"I am not worried about inflation right now, I am worried about an underemployed workforce in America," said Fisher.

(Reporting by Sarah Marsh and Reinhard Becker in Berlin, Andrea Hopkins and Jeffrey Hodgson in Toronto; Writing by Ann Saphir; Editing by Vicki Allen)

• A Permanent Economic Emergency