WASHINGTON — U.S. home prices jumped 10.9 percent in March compared with a year ago, the most since April 2006.

A growing number of buyers are bidding on a tight supply of homes, driving prices higher and helping the housing market recover.

The Standard & Poor’s/Case-Shiller home price index released today also showed that all 20 cities measured by the report posted year-over-year gains for the third straight month.

And prices rose in 15 cities in March from February. That’s up from only 11 in the previous month. The monthly figures aren’t seasonally adjusted and may reflect the beginning of the spring buying season.

Prices rose in Phoenix by 22.5 percent over the past 12 months, the biggest gain among cities. It was followed by San Francisco at 22.2 percent and Las Vegas at 20.6 percent.

New York City had the smallest year-over-year increase at 2.6 percent, followed by Cleveland at 4.8 percent.