Home prices increased for the second consecutive month, according to the S&P/Case-Shiller Home Price Index.

Average monthly home prices increased 2.2 percent in May for the index's 10- and 20-city composites.

Since May 2011, home prices fell 1 percent and 0.7 percent for the 10-city and 20-city composite, respectively.

"We have observed two consecutive months of increasing home prices and overall improvements in monthly and annual returns," David Blitzer, chairman of the Index Committee at S&P Dow Jones Indices, said in a statement. "However, we need to remember that spring and early summer are seasonally strong buying months so this trend must continue throughout the summer and into the fall."

The 10- and 20-city composites had annual rates of decline of 1 percent and 0.7 percent, respectively, compared with May 2011. Only three cities (Boston, Charlotte and Detroit) had annual returns worsen in May.

"While still negative, these annual changes are the best we've since in at least 18 months," Blitzer said.

Zillow's chief economist, Stan Humphries, said May was "a good month" for housing with existing home sales almost 10 percent higher than year-ago levels, supply shortages in some markets and a declining mix of foreclosure resales that affect the Case-Shiller index.

"Moreover, all of the data that has come out in the two months since May indicates that the housing market is continuing to slowly heal," Humphries said.

Although he expects the index will show monthly declines in the latter half of the year, Humphries said that was "a function of seasonality" of a rising share of foreclosures in overall sales, which decline in the fall and winter.

"Overall, we remain cautiously optimistic that home values are at a bottom nationally," he said, "even while our expectations for price appreciation in the next couple of years are muted."