A closely watched inflation indicator ticked up unexpectedly in December, stoking concerns that consumers and the U.S. economy continue to face challenges from rising prices.

The Producer Price Index, which measures changes in U.S. wholesale prices paid by businesses, increased by 0.5% in December, according to data released Friday by the U.S. Bureau of Labor Statistics.

December’s data marks the index’s highest rate in the last three months. The jump can be largely attributed to a 0.7% rise in service prices, the BLS said, noting this is its largest increase since July.

The bureau said the majority of the jump in service prices is from trade services, with over 40% from a “rise in margins for machinery and equipment wholesaling.”