U.S. retail sales fell 2.5 percent last week from a year earlier as fewer people visited stores, retail tracking firm ShopperTrak said on Thursday, the latest signal that this holiday season's growth may not be as robust as some had anticipated.

ShopperTrak, which monitors the number of people walking into stores across the United States, said foot traffic fell 3.3 percent in the week ended December 22 from last year.

However, sales increased 39.1 percent and traffic rose 32.0 percent compared to the previous week, as procrastinators rushed to finish last-minute shopping, ShopperTrak said.

Last week, ShopperTrak lowered its holiday season forecast, calling for sales in November and December to increase only 2.5 percent from 2011, down from 3.3 percent.

The Saturday before Christmas, December 22, was the second-busiest retail sales and foot traffic day of the year, behind Black Friday on November 23, the traditional start of the holiday shopping season, ShopperTrak said.

Shopping on the day after Christmas probably was not as strong this year as it was in 2011, ShopperTrak said. This year, December 26 fell on a Wednesday, and many people likely were back at work. Last year, the day after Christmas came on a Monday.

In contrast, data released by IBM Benchmark showed online sales on December 26 rose 40.4 percent over 2011 and that Apple Inc's iPad drove more retail shopping than any other device.

Traffic via iPad was at 10 percent versus 8.8 percent and 5.7 percent for Apple's iPhone and Google Inc's Android platform respectively, according to the findings by IBM Benchmark, which analyzes data from 500 retailers nationwide.

(Reporting by Nivedita Bhattacharjee in Chicago; Editing by Dale Hudson)