Sales at U.S. retailers rose slightly less than expected in December of 0.8 percent in output at the nation's factories, mines and utilities in December, but cold weather was a factor. Despite the modest rise in sales in December, a key shopping month, sales for all of 2010 reversed two years of contraction with the biggest gain in more than a decade. "We are gathering momentum," said Swiss Re chief economist Kurt Karl, adding that a three-month moving average of retail sales was "really strong." Stock index futures added to losses after the sales data, which helped lift prices for U.S. government debt. The euro gained against the dollar. The reports are unlikely to budge the U.S. Federal Reserve from its $600 billion (378 billion pounds) bond buying program aimed at accelerating growth and lowering the lofty jobless rate. Retail sales climbed 0.6 percent, advancing for the sixth straight month as sales declines at electronics and general merchandise stores were offset by gains in gasoline and building materials sales, the Commerce Department said. Analysts had expected sales to rise 0.8 percent. Excluding autos, sales rose 0.5 percent. Analysts had forecast a 0.7 percent increase.