House prices suffered their worst decline since at least 1987 in the second quarter from a year earlier as the housing downturn has deepened, according to the S&P/Case-Shiller U.S. National Home Price Index.

The national index, which dates from 1987, fell 3.2 percent to 183.89 last quarter from 189.93 in the same period in 2006, S&P said in a statement.

The pullback in the U.S. residential real estate market is showing no signs of slowing down, Robert Shiller, chief economist at MacroMarkets LLC, said in the statement.

Separate indexes of house prices across 10 and 20 major metropolitan areas also dropped. The Composite-20 index fell 3.5 percent in June from a year ago to 199.18, while the index measuring 10 regions slipped 4.1 percent to 217.07.