The unemployment rate surged to 5 percent in December as the nation added only 18,000 jobs, the smallest monthly increase in four years, the Labor Department reported on Friday. Investors responded by sending the stock market down sharply.

Economists treated the report as the most powerful evidence to date that the United States could well be falling into a recessionary downturn in economic activity.

“This is unambiguously negative,” said Mark Zandi, chief economist at Moody’s Economy.com. “The economy is on the edge of recession, if we’re not already engulfed in one.”

The swift deterioration in the job market resonated as a warning sign that troubles once confined to real estate and construction are now spilling over into the broader economy, threatening the ability of American consumers to keep spending with customary abandon.