Single-family home prices unexpectedly climbed in April from March, driven by a final sales push before tax credits expired, but signs of a sustained recovery have yet to emerge, Standard & Poor's/Case Shiller home price indexes showed on Tuesday.

Inventory data and foreclosure activity have not shown any signs of improvement, David Blitzer, chairman of S&P index committee, which publishes the price indexes. Consistent and sustained boosts to economic growth from housing may have to wait to next year.

The S&P composite index of home prices in 20 metropolitan areas for April rose 0.4 percent on a seasonally adjusted basis, surprising economists who had forecast a 0.1 percent decline in a Reuters survey.

For March, the seasonally adjusted index was downwardly revised to a 0.2 percent drop, compared with unchanged previously.

On an unadjusted basis, prices gained 0.8 percent in April following March's 0.5 percent drop. A 0.2 percent rise was forecast in a Reuters poll.

The 20-city index rose 3.8 percent in April from a year earlier, topping the expected 3.4 percent increase.

Most of it could be attributable to the (federal) tax credit, which pulled forward demand. Now demand is falling away, said Kevin Logan, chief U.S. economist at HSBC Securities.

Home sales have fallen precipitously in the weeks since the April 30 end of tax credits of up to $8,000, which pushed sales forward as buyers raced to lock in the incentive.

Other reports have shown sales of new homes sank by a record 32.7 percent in May to the lowest level since record keeping began in the early 1960s, and existing home sales unexpectedly fell 2.2 percent in May.

Applications to buy homes hover at 13-year lows.

The tax credit fired up sales but also boosted inventory, adding to supply pressures already in place from record foreclosures.

People have been listing their houses as the tone of the market has improved, said Pierre Ellis, senior economist at Decision Economics. That may tend to limit price increases, and housing stability is critical for consumer spending, given stock market uncertainty.

Unemployment also remains stubbornly high at 9.7 percent and lending standards are tight, making homeownership unattainable for many borrowers.

Still, affordability is high, with prices down around 30 percent on average from 2006 peaks and mortgage rates touching record lows near 4-3/4 percent.

Home prices in April were at levels similar to late summer and early autumn of 2003, S&P said.

The tax credit-driven gains in April spurred month-to-month increases in 18 cities, compared with six in March. Only Miami and New York saw price erosion last month -- 0.8 percent and 0.3 percent, respectively.

Prices rose by at least 1 percent in 11 of the metro areas in April from March. New York posted a new cyclical low and has now fallen 21.7 percent from the June 2006 peak.

(Additional reporting by Richard Leong and Ellen Freilich, Editing by W Simon )

S&P