Jan 9 (Reuters) - U.S. retail sales rose 2.5 percent during the 2012 holiday season, helped by a late surge in store visits in the days right before Christmas, retail tracking firm ShopperTrak said on Wednesday. ShopperTrak, which monitors the number of people walking into stores across the United States, said foot traffic also rose 2.5 percent in November and December. "Heavy discounting by retailers at the end of December led to increases in shopper activity," ShopperTrak founder Bill Martin said in a statement. In mid-December, after signs of slowdown in retail sales after the season got off to a strong start at Thanksgiving, ShopperTrak lowered its forecast, calling for sales to increase 2.5 percent from 2011, compared to an earlier forecast of a 3.3 percent jump. The National Retail Federation, which will publish its holiday sales results next week has forecast an increase of 4.1 percent.