Ben S. Bernanke, the Federal Reserve chairman, warned Wednesday that an aging American population posed fiscal challenges for the United States.

In a rare foray into the realm of fiscal policy, Mr. Bernanke said the United States “must begin now to prepare for this coming demographic transition.”

Mr. Bernanke, speaking before the Dallas Regional Chamber, said that the economy was still fragile, and he made it clear that he did not expect the federal government to raise taxes or cut spending anytime soon. But he spoke about the budget pressures posed by Social Security and Medicare with greater urgency than he has in the past.

“The arithmetic is, unfortunately, quite clear,” Mr. Bernanke said. “To avoid large and unsustainable budget deficits, the nation will ultimately have to choose among higher taxes, modifications to entitlement programs such as Social Security and Medicare, less spending on everything else from education to defense, or some combination of the above. These choices are difficult, and it always seems easier to put them off — until the day they cannot be put off any more.”