U.S. home prices rose at a healthy clip in August from a year ago, a trend that is thwarting many would-be buyers and potentially slowing sales.

The Standard & Poor's CoreLogic Case-Shiller national home price index increased 6.1 percent in August from a year earlier. That's higher than the 5.9 percent annual gain in July. In 9 of the 20 cities tracked by the index, yearly price gains in August were faster than in July.

Fewer Americans are selling their homes, and builders aren't putting up enough new houses to meet burgeoning demand. That has forced home buyers into bidding wars, lifting prices.

Mortgage rates remain at historically low levels, making it easier to afford higher costs. That has resulted in rising prices even as sales have slipped this year.