WASHINGTON—Compensation for American workers accelerated in 2017, signaling that historically low unemployment might be starting to put upward pressure on wages and benefits.

The employment-cost index, a measure of wages and benefits for civilian workers, rose 2.6% over the past year, matching the largest annual increase since 2015, when compensation also increased at a 2.6% rate, according to Wednesday’s Labor Department report. Growth in the index hasn’t exceeded 2.6% since 2008.

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