Home prices in most major cities in the US are rising for the first time in eight months, boosted by an annual wave of spring buying. Analysts cautioned that the increases may be temporary and don't signal a rebound for the depressed home market.

Prices rose in 13 of the 20 cities tracked by the Standard & Poor's/Case-Shiller home-price index, accoding to the April report released Tuesday. The sharpest increases were in Washington DC. The next largest were in San Francisco, Atlanta and Seattle.

The index covers metro areas that together make up about 50 percent of US househoulds. It measures sale prices of select homes in those cities compared with prices in January 2000. It then provides a three-month average. The April data is the latest available.

Last year, a tax credit for first-time buyers helped boost prices. They rose nearly 4 percent from April through July before falling more than 7 percent this winter to record lows. Prices in metro areas sank in March to their lowest level since 2002.

The 0.7 percent increase in April was the first rise since July. The positive data came with a caveat: The figures weren't adjusted for seasonal factors, such as the buying that normally picks up in spring. Once the numbers are adjusted, priced actually fell in April.

David M. Blitzer, chairman of S&P's index committee, said the rise in the index was a welcome shift from recent months. But he noted that much of the improvement was likely due to the start of the buying season.

"It is much too early to tell if this is a turning point or simply due to some warmer weather," he said.

One bright spot: Even when adjusted for seasonal factors, prices rose in some markets that had been pummeled by slumping sales, Atlanta, Minneapolis, Phoenix and Portland, Oregon.

On the other hand, prices in six areas have reached their lowest levels in nearly four years: Charlotte, Chicago, Detroit, Las Vegas, Miami and Tampa.

In Atlanta, more than a quarter of homes sold in the past year were discounted: They sold for an average 9 percent less than they'd been listed for, according to data analyzed by the housing website Trulia.com.