U.S. consumer prices unexpectedly declined in May, a sign that inflation pressure remain muted.

Consumer Price Index (CPI), a main gauge of inflation, fell 0.1 percent in May on a seasonally adjusted basis, compared to 0.2 percent increase in the previous month, said the Labor Department on Wednesday.

On a year-on-year basis, the index increased 1.9 percent, down from the 2.2 percent increase in April.

Food index rose 0.2 percent in May, the same growth as in April, while the energy index fell 2.7 percent in the month, after rising 1.1 percent in April.

Excluding the volatile food and energy categories, the so-called core CPI went up 0.1 percent in May on a seasonally adjusted basis, and was up 1.7 percent over the year, also lower than the 1.9 percent growth in April.

The Federal Reserve is expected to raise interest rates later on Wednesday, but the softness of the inflation might lead some Fed officials to give a second thought about the pace of interest rate hikes.