US home prices rose in March, but the gains are slowing as fewer Americans can afford to buy, a closely watched report said Tuesday. The Standard & Poor's/Case-Shiller 20-city home-price index rose 12.4 percent in March compared to a year earlier. While healthy, the March increase is smaller than those from February and January. Home prices rose in 19 of the 20 cities in March compared with the previous month, with only New York City registering a slight decline. The index is not adjusted for seasonal variations, so the March gains likely reflected the warmer weather after a severe winter. Price gains last year and higher mortgage rates have reduced affordability, limiting the number of potential buyers, which has slowed this year's price increases.