NEW YORK CITY—“Home price increases appear to be unstoppable,” says S&P Dow Jones Indices' David M. Blitzer in reference to the latest S&P CoreLogic Case-Shiller Indices, released Tuesday. One of the three indices, the US National Home Price NSA Index, covering all nine US census divisions, reached a new high with a 6.1% annual gain in August, up from 5.9% in July. The NSA Index has hit new peaks in each of the past nine months.

However, even as Blitzer cites the current momentum, the managing director and chairman of the index committee at S&P Dow Jones Indices also says that the monthly gains won't occur indefinitely. “Measures of affordability are beginning to slide, indicating that the pool of buyers is shrinking,” says Blitzer. “The Federal Reserve is pushing short-term interest rates upward and mortgage rates are likely to follow over time, removing a key factor supporting rising home prices.”

Both of the other two Case-Shiller indices, the 10-City Composite and 20-City Composite, saw higher annual gains in August compared to the previous month. The 10-City Composite annual increase came in at 5.3%, up from 5.2% the previous month, while the 20-City Composite posted a 5.9% year-over-year gain, up from 5.8% in July.