FALLUJAH, Iraq, Nov. 13 -- As senior Iraqi officials declared Fallujah liberated, U.S. forces on Saturday continued intense combat operations aimed at securing the last section of the city from an insurgent force fighting with surprising discipline, organization and the trappings of a professional army, American commanders said. In the southernmost section of Fallujah, where a showdown still loomed, U.S. soldiers discovered an underground bunker and steel-enforced tunnels connecting a ring of houses filled with weapons, medical supplies and bunk beds.