Consumer prices declined in May, reflecting a big drop in energy prices and smaller declines in a number of other areas. It was the second monthly decline in the past three months and underscores how inflation has been a no-show in the slow-growing U.S. economy.

Consumer prices edged down 0.1 percent last month following a small 0.2 percent increase in April, the Labor Department reported. Prices had fallen 0.3 percent in March. In addition to a drop in energy costs last month, the price of clothing, airline fares and medical care also declined.

Core inflation, which excludes energy and food, rose a slight 0.1 percent in May.

"From out of nowhere we have now had three months of unusual weakness in underlying prices," said Paul Ashworth, chief economist at Capital Economics. He said the inflation slowdown was occurring at the same time that the unemployment rate has fallen to a 16-year low.

"It won't stop the Fed from hiking interest rates later today, but it increases the downside risks to our forecast that there will be a further two rate hikes in the second half of this year," he said in a research note.

Over the past 12 months, consumer prices are up 1.9 percent while core inflation has risen 1.7 percent. The Federal Reserve seeks to manage interest rates to promote moderate annual gains in inflation of 2 percent. The Fed's preferred measure of inflation, tied to consumer spending, has been below 2 percent for five years and in recent months slipped back a bit. However, Fed officials have said they believe the recent slowdown in price gains reflected transitory factors and they expect inflation will resume moving toward 2 percent as low unemployment helps to boost wages. Unemployment in May dipped to 4.3 percent.

In May, food costs edged up a tiny 0.2 percent while energy costs fell 2.7 percent, led by a 6.2 percent drop in the price of gasoline. Over the past 12 months, food costs are up just 0.9 percent while energy prices have risen 5.4 percent.