June 29 (Reuters) - U.S. single-family home prices in 20 key urban markets rose in April from a year earlier by the most in over 15 years, a closely watched survey said on Tuesday. The S&P/Case Shiller composite index of 20 metropolitan areas gained 14.9% through the 12 months ended in April, the largest annual price increase since December 2005. A Reuters poll of economists had forecast a 14.5% increase. On month-to-month basis, the 20-city composite index rose 1.6% from March. Economists polled by Reuters had been expecting a 1.7% increase. Home prices have surged nationwide in large part due to limited supply. Last week, the Commerce Department reported that sales of new U.S. single-family homes fell to one-year low in May as expensive raw materials such as lumber continue to increase prices of newly constructed houses. Also, the National Association of Realtors said sales of previously owned homes fell for a fourth consecutive month in May. (Reporting by Evan Sully Editing by Chizu Nomiyama) Our Standards: The Thomson Reuters Trust Principles.