NEAR FALLUJAH, Iraq -- U.S. troops backed by thunderous air and artillery barrages launched a ground offensive Monday to seize key insurgent strongholds inside Fallujah, the city that became Iraq's major sanctuary for Islamic extremists who fought Marines to a standstill last April.
Two Marines were killed when their bulldozer flipped over into the Euphrates River near Fallujah, and a military spokesman estimated 42 insurgents were killed across the city in bombardment and skirmishes before the main assault began.
Hours after starting the offensive, U.S. tanks and Humvees from the 1st Infantry Division entered the northeastern Askari neighborhood, the first ground assault into an insurgent bastion.
In the northwestern area of the city, U.S. troops advanced slowly after dusk on the Jolan neighborhood, a warren of alleyways where Sunni militants have dug in. Artillery, tanks and warplanes pounded the district's northern edge, softening the defenses and trying to set off any bombs or booby traps planted by the militants.
Heavy firing continued into the predawn hours today, and residents reached by satellite telephone reported the constant drone of warplanes overhead.
A U.S. jet fired an air-to-ground missile at a building late Monday from which U.S. and Iraqi forces had taken fire, the U.S. command said. The building was destroyed.
U.S. troops cut off electricity to the city, and most private generators were not working. Residents said they were without running water and were worried about food shortages because most shops in the city have been closed for the past two days.
Masked insurgents roamed Fallujah streets throughout the day. One group of four fighters, two of them draped with belts of ammunition, moved through narrow passageways, firing on U.S. forces with small arms and mortars. Mosque loudspeakers blared, "God is great, God is great."
Just outside the Jolan and Askari neighborhoods, Iraqi troops deployed with U.S. forces took over a train station after the Americans fired on it to drive off fighters.
The top U.S. commander in Iraq, Gen. George Casey, predicted a "major confrontation" in the operation he said was called "al-Fajr," Arabic for "dawn." He told reporters in Washington that 10,000 to 15,000 U.S. troops along with a smaller number of Iraqi forces were encircling the city.
The offensive is considered the most important military effort to re-establish government control over Sunni strongholds west of Baghdad before elections in January.
"One part of the country cannot remain under the rule of assassins ... and the remnants of Saddam Hussein's regime," Defense Secretary Donald H. Rumsfeld said. He predicted "there aren't going be large numbers of civilians killed and certainly not by U.S. forces."
About 3,000 insurgents were barricaded in Fallujah, U.S. commanders have estimated.
Casey said 50 to 70 percent of the city's 200,000 residents have fled. The numbers are in dispute, however, with some putting the population at 300,000. Residents said about half that number left in October, but many drifted back.
U.S. commanders have avoided any public estimate on how long it may take to capture Fallujah, where insurgents fought the Marines to a standstill last April in a three-week siege.
Arab leaders were muted in their response to the offensive. Media attention focused on ailing Palestinian leader Yasser Arafat, which may explain in part why the start of the Fallujah campaign elicited none of the uproar that met the American attempt to storm the insurgent stronghold in April.