WASHINGTON — U.S. home prices are falling again in most major cities after posting small gains over the summer and spring, the latest evidence that the troubled housing market won’t recover anytime soon.

The Standard & Poor’s/Case-Shiller index released Tuesday showed prices dropped in September from August in 17 of the 20 cities tracked.

That was the first decline after five straight months in which at least half the cities in the survey showed monthly gains.

A separate index for the July-September quarter shows prices were mostly unchanged from the previous quarter.

Prices rose in New York, Portland, Ore. and Washington.