(Corrects headline to May from April) (Reuters) - U.S. single-family home prices in 20 key urban markets rose in May from a year earlier at the fastest pace in more than 16 years, a closely watched survey said on Tuesday. The S&P/Case Shiller composite index of 20 metropolitan areas gained 17.0% through the 12 months ended in May, the largest annual price increase since August 2004. A Reuters poll of economists had forecast a 16.4% increase. On a month-to-month basis, the 20-city composite index rose 1.8% from April. Economists polled by Reuters had been expecting a 1.6% increase. Reporting by Evan Sully; Editing by Peter Graff Our Standards: The Thomson Reuters Trust Principles.