WASHINGTON—U.S. labor costs rose at the slowest pace in at least three decades in the spring, a sign of persistently sluggish wage growth that could weigh on the Federal Reserve's decision to raise short-term interest rates.

The employment-cost index, a broad measure of workers' wages and benefits, climbed a seasonally adjusted 0.2% in the second quarter from the first quarter, the Labor Department said Friday. That marked the smallest quarterly gain since record keeping began in 1982.

Economists surveyed by The Wall Street Journal expected a 0.6% increase.

Civilian worker wages and salaries, reflecting more than two-thirds of employee costs, grew 0.2% in April through June, also the smallest gain on record. Benefits climbed 0.1%.

The latest compensation gains were due entirely to higher employment costs for government workers. Compensation for private sector workers was flat in the spring, the first time on record that that category failed to rise.

Compensation costs can ebb and flow from quarter to quarter. Over a broader period, wages and benefits are growing modestly. Total compensation grew 2.0% in the second quarter compared to a year earlier. That was a slower pace than the 2.6% year over year growth in the winter.

The Federal Reserve is monitoring wage figures as it moves closer to raising interest rates from near zero, their level since late 2008. An acceleration in wages would signal the labor market is finally close to healthy, six years after the recession, and could nudge the Fed to act sooner rather than later to avoid an overheating economy.

But the latest slowdown suggests slack remains in the jobs market and could bolster the case for officials to wait longer to raise rates.

Economists have long anticipated a pickup in wages as the labor market tightens, but previous jumps have been short-lived. Another measure of wages, gleaned from the Labor Department's monthly jobs report, has disappointed recently. The average hourly earnings of private-sector workers-which exclude benefits--rose a modest 2% in the year through June.

The labor market otherwise appears to be progressing as hiring remains steady. The unemployment rate fell to 5.3% in June - down from a peak of 10% in October 2010 and within spitting distance of the 5% to 5.2% range the Fed considers the economy's long-run average.

But the persistent lack of momentum in wage growth suggests slack remains, likely due to the millions of Americans who are jobless or aren't even looking for work even though they are of prime working age. A broader measure of unemployment-which takes into account those Americans as well as part-time employees who would prefer full-time work-stood at 10.5% in June. Just before the recession, that figure stood at 8.4%.

On Wednesday, Federal Reserve officials released a statement after their latest policy meeting suggesting they see continued progress in the labor market, and suggested the central bank could move as early as September to start raising short-term interest rates.

"The Committee anticipates that it will be appropriate to raise the target range for the federal funds rate when it has seen some further improvement in the labor market and is reasonably confident that inflation will move back to its 2 percent objective over the medium term," the central bank said in a policy statement.

The economy overall regained steam in the second quarter but continued to underperform. Gross domestic product grew at a 2.3% annual pace in the spring, up from the first quarter's 0.6% pace, the Commerce Department said Thursday.

Friday's report showed private-sector compensation costs were flat in the second quarter after rising 0.7% in the first quarter. Compensation costs for state and local governments rose 0.6% in the second after climbing 0.5% in the first.

Write to Josh Mitchell at joshua.mitchell@wsj.com

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