TAMPA - Government incentives and low mortgage rates boosted home sales, but they weren't enough to push up sales prices.

March's sales prices fell 3.5 percent in the Tampa metro area, compared with the same month last year, according to the Standard & Poor's/Case-Shiller 20-city index released Tuesday. Prices fell 0.1 percent from February.

Nationally, prices fell 2 percent from last year and 1.8 percent from the previous month.

That data prompted the co-creator of the Case-Shiller index to raise concerns that the worst may be ahead. He points to weak job growth, tight credit and many more foreclosures on the horizon.

"I'm worried still about the risk of a double-dip," economist Robert Shiller said in a statement. The month-to-month drop from February to March marked the sixth-straight decline. Prices in 13 of the cities fell. Only six metro areas recorded price gains. One, Boston, came in flat.

In the first quarter of 2010, U.S. home prices fell 3.2 percent compared with the fourth quarter.

Prices remain nearly 31 percent below their July 2006 peak. However, they have risen nearly 3 percent from their April 2009 bottom.

Miami, the only other Florida city on the list, saw prices drop 1.7 percent from last year and 0.9 percent from the previous month.

The news was particularly disappointing following February's national increase of 0.6 percent.

The index also follows Monday's report from the National Association of Realtors that showed existing home sales rose 7.6 percent in April.

The Tampa-St. Petersburg-Clearwater metro area fared even better, according to Florida Realtors. Home sales rose 27 percent. The association said median sales price fell 2 percent from last year to $132,900.

Information from The Associated Press was used in this report. Reporter Shannon Behnken can be reached at (813) 259-7804.