U.S. forces stormed into the western outskirts of Fallujah early Monday, seizing the main city hospital and securing two key bridges over the Euphrates river in what appeared to be the first stage of the long-expected assault on the insurgent stronghold. The U.S. military reported Monday its first casualties of the offensive_ two Marines killed when their bulldozer flipped over into the Euphrates River. An AC-130 gunship raked the city all night long with cannon fire as heavy explosions from U.S. artillery continued into Monday morning. Four 500-pound bombs were dropped over Fallujah before dawn. Orange fireballs from high explosive airbursts could be seen above the rooftops.