Low inventory and mortgage rates pushed home-price gains to a 30-month high in December, according to the S&P/Case-Shiller U.S. National Home Price Index.

The index, which measures all nine U.S. census divisions, found that home prices rose 5.8 percent year over year, up from November's 5.6 percent annual gain. The December rise was the highest annual increase since June 2014, when it rose 6.3 percent vs. to June 2013.

A poll by Reuters showed that the index was expected to rise 5.3 percent in December, after rising by the same amount a month earlier.

The S&P/Case Shiller 20-city composite index, which tracks the nation's largest cities, gained 5.6 percent year over year, up from 5.2 percent the previous month. Seattle, Portland, Oregon, and Denver once again topped the charts with the largest year-over-year gains. Seattle continued to lead the pack, rising at an annualized rate of 10.8 percent.

Of the nation's 20 largest cities, seven reached their all-time highs in December: Seattle, Portland, Denver, Boston, Charlotte, North Carolina, San Francisco and Dallas.

"Home prices continue to advance, with the national average rising faster than at any time in the last two-and-a-half years," David Blitzer, managing director and chairman of the index committee at S&P Dow Jones Indices, said in a press release.

And while rising home prices can trigger concerns about inflation, Blitzer said the speed at which they are growing "is not alarming." He cited low inventory as a factor for the rise.

"While sales of existing single-family homes passed five million units at annual rates in January, the highest since 2007, the inventory of homes for sales remains quite low with a 3.6 month supply," he said.

Blitzer noted that new home sales were higher than in recent years, at 555,000 in 2016, but still under the average pace of 700,000 since 1990.

Mortgage rates also had an impact on the rise, Blitzer said: "A 30-year fixed rate mortgage today is 4.2 percent compared to the 6.4 percent average since 1990."

Correction: This story was revised to correct that the 30-month high reflected the annual percentage price rise.