The decline in prices of existing U.S. single-family homes accelerated in August and fell at their fastest pace since 1991, according to the Standard & Poor's/Case Shiller national home price index.

S&P said its composite month-over-month index of 10 major metropolitan areas declined 0.8 percent in August to 214.35, for a 5 percent year-over-year drop, S&P said in a statement on Tuesday.