NEW YORK (Reuters) - U.S. single-family home prices were unchanged in May, the first time in nearly a year they have not fallen on a monthly basis, though prices were still down compared to a year earlier, a closely watched survey said Tuesday.

The S&P/Case Shiller composite index of home prices in 20 metropolitan areas held steady on a seasonally adjusted basis and in line with economists' expectations, according to a Reuters poll.

The last time the seasonally adjusted index rose month-over-month was in June 2010, shortly after the home buyer tax credit expired.

On a non-seasonally adjusted basis, the index rose 1 percent, the second month of gains in a row, the report said.

"This is a seasonal period of stronger demand for houses, so monthly price increases are to be expected," David Blitzer, chairman of the index committee at Standard & Poor's, said in a statement.

"The concern is that much of the monthly gains are only seasonal."

Prices in the 20 cities fell 4.5 percent year-over-year, which was also in line with forecasts.

(Reporting by Leah Schnurr, Editing by Chizu Nomiyama)