NEW YORK -- Prices of existing U.S. single-family houses extended their slide across the country in May, marking the 18th consecutive decline in the growth rate, according to an index of major metropolitan areas.

The composite month-over-month Standard & Poor's/Case-Shiller Home Price Index of 10 metropolitan areas declined 0.3 percent to 218.37, bringing the year-over-year fall to 3.4 percent, S&P said in a press release.

The composite month-over-month Standard & Poor's/Case-Shiller Home Price Index of 20 metro areas also showed a 0.2 percent drop, to a 200.04 reading, or a 2.8 percent year-over-year loss.

"At a national level, declines in annual home price returns are showing no signs of a slowdown or turnaround," Robert J. Shiller, chief economist at MacroMarkets LLC, said in the release.

Year-over-year price returns are continuing to either move deeper into negative territory or experience persistent diminishing returns although in May and April eight of the 20 markets showed positive monthly growth rates, he said.

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