U.S. single-family home prices rose in August on a year-over-year basis but fell short of expectations, a closely watched survey said on Tuesday.
The S&P/Case Shiller composite index of 20 metropolitan areas gained 5.6 percent in August over last year, the slowest year-on-year increase since November 2012, slightly below a Reuters poll of economists that forecast a gain of 5.8 percent.
To Robert Shiller, the co-founder of the Case-Shiller index and a professor of economics at Yale University, the latest data is open to interpretation.
"I've learned over the years forecasting home prices that the latest month is very important, but the trend is also important ... so what we've seen nationally is big increases a few months ago and now a softening," Shiller said Tuesday on CNBC's "Squawk on the Street." "So what that means, the latest month suggests price declines might be coming, but if you mix that in, it's kind of an ambiguous situation. Maybe they won't do much of anything."
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On a seasonally adjusted monthly basis, prices in the 20 cities dipped 0.1 percent for the month. A Reuters poll of economists had forecast an increase of 0.1 percent.
Non-seasonally adjusted prices rose 0.2 percent in the 20 cities on a monthly basis, disappointing expectations for a 0.5 percent rise.
"The deceleration in home prices continues," David Blitzer, chairman of the index committee at S&P Dow Jones Indices, said in a statement.
"Despite the weaker year-over-year numbers, home prices are still showing an overall increase, as the National Index increased for its eighth consecutive month."
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A broader measure of national housing market activity that S&P/Case-Shiller is now releasing on a monthly basis rose at a slower pace year over year, coming in at 5.1 percent.
The seasonally adjusted 10-city gauge fell 0.2 percent in August versus a 0.5 percent decline in July, while the non-adjusted 10-city index rose 0.2 percent in August compared to a 0.6 percent rise in July.
There is a chance that home prices could go negative, if only for a brief amount of time, Shiller said. After all, the housing recovery has been on a tear over the past few years, as the U.S. government sought to stimulate the economy through quantitative easing and other programs, he noted. It's to be expected people would question housing, though, as the Fed finally considers ending QE, he said.
—CNBC's Drew Sandholm contributed to this report.