(Reuters) - U.S. single-family home prices rose for a sixth month in a row in July, though the improvement was not as strong as expected, a closely watched survey showed on Tuesday.

The S&P/Case Shiller Composite Index of 20 metropolitan areas gained 0.4 percent in July on a seasonally adjusted basis, shy of economists' forecasts for 0.9 percent, according to a Reuters poll.

On a nonadjusted basis, prices fared better, rising 1.6 percent.

Six years after its collapse, economists believe the housing market has turned a corner. Recent data show home resales and groundbreaking on new properties rose in August, while business sentiment among homebuilders picked up to a more than six-year high this month.

The home price data confirmed "recent good news" about the sector, David Blitzer, chairman of the index committee at Standard & Poor's, said in a statement.

"All in all, we are more optimistic about housing. Upbeat trends continue," said Blitzer.

U.S. stock index futures held gains immediately after the data, while long-dated Treasury bond prices trimmed gains, and the dollar extended losses slightly against the euro.

"This shows a continual recovery in the housing market, which continues to progress," said Mike Gibbs, co-head of the equity advisory group at Raymond James in Memphis, Tenn.

Compared to a year ago, prices in the 20 cities were up 1.2 percent, beating expectations for 1 percent. It was the second month in a row year-over-year prices have risen.

Four cities had prices that were lower than a year ago, with Atlanta faring the worst, down nearly 10 percent. Hard-hit Phoenix continued its rebound to gain 16.6 percent.

U.S.