WASHINGTON - Federal Reserve Chairman Ben S. Bernanke warned yesterday that the nation's economy could be gravely hurt if Social Security and Medicare weren't revamped.

He urged lawmakers to tackle the nation's thorny fiscal issues sooner rather than later.

"If early and meaningful action is not taken, the U.S. economy could be seriously weakened," Bernanke told the Senate Budget Committee.

It marked the Fed chief's most forceful warning to date on the potential problems facing the United States with the looming retirement of 78 million baby boomers.

"The longer we wait, the more severe, the more draconian, the more difficult the objectives are going to be. I think the right time to start is about 10 years ago," he told lawmakers.

Bernanke, who took over the Fed last February, made his case in somewhat starker terms than had his predecessor, Alan Greenspan, who repeatedly sounded the alarm during his tenure about the dangers to the economy from exploding entitlement costs, some economists said.

Bernanke said yesterday that, without policy changes by Congress and the White House, rising federal budget deficits were likely to increase the amount of debt outstanding to unprecedented levels in the years ahead.

That could propel interest rates for consumers and businesses upward, which would be a worrisome development, he said.

"Thus, a vicious cycle may develop in which large deficits lead to rapid growth in debt and interest payments, which, in turn, adds to subsequent deficits," he said.

Ultimately, a big expansion of the nation's debt "would spark a fiscal crisis, which could be addressed only by very sharp spending cuts, or tax increases, or both," Bernanke warned.

Sen. Judd Gregg of New Hampshire, the committee's top-ranking Republican, called Bernanke's warning "right on, and a clarion call that I hope folks will listen to."

Last year, the budget deficit totaled $248 billion, a four-year low. Bernanke noted the improvement, but likened it to a "calm before the storm."

Spending on entitlement programs will begin to climb quickly during the next decade, he said. Federal spending for Social Security, Medicare and Medicaid will total about 15 percent of the gross domestic product by 2030, compared with roughly 8.5 percent of GDP in 2006, he said.