U.S. consumer prices fell for the first time in nine months in December amid a plunge in the cost of gasoline, but underlying inflation pressures remained firm as rental housing and healthcare costs rose steadily.

The Labor Department said on Friday its Consumer Price Index dipped 0.1 percent last month, the first drop and weakest reading since March. The CPI was unchanged in November. In the 12 months through December, the CPI rose 1.9 percent after increasing 2.2 percent in November.

Excluding the volatile food and energy components, the CPI increased 0.2 percent, advancing by the same margin for a third straight month. In the 12 months through December, the so-called core CPI rose 2.2 percent, matching November's increase.

December's inflation readings were in line with economists' expectations. The Federal Reserve, which has a 2 percent inflation target, tracks a different measure, the core personal consumption expenditures (PCE) price index, for monetary policy.

The core PCE increased 1.9 percent year-on-year in November after rising 1.8 percent in October. It hit 2 percent in March for the first time since April 2012.

A sharp decline in oil prices amid an oversupply and slowing global economic growth is keeping overall inflation in check. Lower oil prices are also filtering through to core inflation via cheaper airline tickets.

While the Fed has forecast two rate hikes this year, moderate inflation pressures likely support recent statements by several policymakers, including Chairman Jerome Powell, for caution about raising interest rates this year.