U.S. import prices were flat in December as the cost of petroleum fell slightly, the government said Tuesday in a report that showed no signs of imported inflation pressures. November import prices were revised to show a 0.9 percent decline rather than the previously reported 0.6 percent, the Labor Department said. Economists had forecast import prices to rise 0.3 percent in December. For all of 2013, import prices fell 1.3 percent. Import prices excluding petroleum also were flat in December after rising 0.1 percent the previous month. Compared to December 2012, imports excluding petroleum were down 1.3 percent. Export prices rose 0.4 percent last month after a 0.1 percent advance in November, the report said.