Home prices in 20 major US cities were up 12.1 percent in June from a year ago, but the pace of monthly gains slowed from May, according to data released Tuesday. The S&P Case-Shiller 20-city composite index rose 2.2 percent in June from May, according to data that was not seasonally adjusted. In May, the month-on-month gain was a stronger 2.5 percent. Only six cities had prices rising faster in June than the prior month, compared with 10 in May. On a 12-month basis, June\'s 12.1 percent reading was stronger than the 11.9 percent rise in May and analysts\' estimate of 12.0 percent. David Blitzer, chairman of the index committee at S&P Dow Jones Indices, said the monthly data signaled a possible slowing in the pace of price gains, noting rising mortgage interest rates. \"Overall, the report shows that housing prices are rising but the pace may be slowing,\" Blitzer said in a statement, noting weakening gains in 13 cities. \"As we are in the middle of a seasonal buying period, we should expect to see the most gains. With interest rates rising to almost 4.6 percent, home buyers may be discouraged and sharp increases may be dampened.\" Home prices rose 7.1 percent in the second quarter and were up 10.1 percent over the last four quarters as the housing market slowly recovers from the 2006 price crash. The June data were in line with the consensus estimate of analysts. \"The volatility of the numbers mean it is much too soon to call this definitive evidence of a slowdown in the pace of price increases, still less evidence of the impact of the rise in mortgage rates, but we will be watching these data closely over the next few months,\" said Ian Shepherdson, chief economist at Pantheon Macroeconomics. The economist said the numbers probably reflect a small change in the balance of demand and supply, citing a modest rise in the supply of existing homes for sale compared with the pace of sales this year.