WASHINGTON, July 31 (Reuters) - U.S. labor costs increased slightly more than expected in the second quarter as private-sector wage growth picked up, government data showed on Friday.

The Employment Cost Index (ECI), the broadest measure of labor costs, climbed 0.9% last quarter after advancing by the same margin in the January-March quarter, the Labor Department's Bureau of Labor Statistics said. Economists polled by Reuters had forecast the ECI would rise 0.8%.

Labor costs increased 3.4% in the 12 months through June after a similar gain in the year through March. The ECI is viewed by policymakers as one of the better measures of labor market slack and a predictor of core inflation because it adjusts for composition and changes in job quality. 

Job growth accelerated between March and May. The momentum, however, fizzled in June. Economists describe the labor market as being in stuck in a "low hire, low fire" state. Wages and salaries, which account for the bulk of labor costs, increased 0.9% in the second quarter. In the 12 months through June, wages rose 3.2%. Private sector wages and salaries increased 0.9% last quarter after rising 0.7% in the January-March quarter.

The Federal Reserve on Wednesday left its benchmark overnight interest rate in a 3.50%-3.75% range. Three members of the U.S. central bank's policy-setting committee dissented. They "preferred" a quarter-percentage-point hike.