WASHINGTON (AP) — U.S. home prices rose at a healthy pace in December compared with a year ago, driven higher by rising sales and a smaller supply of available homes.

The Standard & Poor's/Case-Shiller 20-city home price index, released Tuesday, rose 6.8 percent in December compared with the same month a year ago. That's up from a 5.5 percent annual gain in November.

Nationwide, the report showed that prices rose 7.3 percent in 2012. That is similar to other home price measures that show a healthy gain last year.

Prices also rose in December compared with a year ago in 19 of the 20 cities tracked by the index. New York was the only metro area to show a decrease.

Steady price increases should help fuel the housing recovery. They encourage more people to buy before prices rise further. Higher prices also build homeowners' wealth, which can spur more spending and economic growth.

Purchases of previously occupied homes rose last year to their highest level in five years. The National Association of Realtors forecasts that sales will rise 9 percent this year. Independent economists have similar forecasts.

At the same time, the number of available homes for sale fell last month to the lowest level in 13 years.

In December, the 20-city index ticked up 0.2 percent from the previous month, reversing November's small decline.

The S&P/Case-Shiller index covers roughly half of U.S. homes. It measures prices compared with those in January 2000 and creates a three-month moving average. The November figures are the latest available.

Despite the increases, prices nationwide are still about 30 percent below the peak they reached at the height of the housing bubble in the summer of 2006. They are now at the same level as in the fall of 2003.