Marching across a bridge spanning the Euphrates River just before dawn today, troops with the 82nd Airborne Division seized control of key sections in this city of 140,000, meeting only sporadic resistance as they fanned out and cleared areas believed to house Iraqi fighters.

Before the troops' advance, U.S. jets, helicopters and Bradley Fighting Vehicles from the Army's 1st Armored Division unleashed a barrage of artillery fire on the city's northern neighborhoods, where most local Baath Party officials are believed to reside.

Two U.S. commanders said at least a dozen Iraqi soldiers and paramilitary fighters were killed during the day's action in Samawah, about 150 miles south of Baghdad. Several civilians were also wounded and sought treatment at an Army aid station north of the river. No U.S. soldiers were reported wounded during the advance and fighting early today. This evening, gunfire sprayed one of the division's camps on the outskirts of the city, injuring five soldiers.

Commanders of the 82nd Airborne Division said the transit corridors through the heart of the city were successfully secured. But the shooting underscored the persistent threat of violence here.

"We now control the lines of communication in and out of Samawah," Maj. Gen. Charles H. Swannick said. "But have we physically been to every part of the city? No."

All week the division has been focusing on transit points running to and from the city in an effort to secure supply routes. Swannick said Highway 8, which runs through the middle of Samawah, was now secure. It provides a more direct route to troops near Baghdad than other roads being used.

The day began for the soldiers at about midnight, when they were taken by convoy to begin their march toward the northern bank of the Euphrates.

Although the preparatory shelling from the air and ground drew small-arms and mortar fire from Iraqi fighters, the trek over the river was quiet, if tense, for U.S. forces.

Swannick likened today's shelling of Iraqi positions to "using a sledgehammer to crack a walnut."

U.S. operations have recently run into trouble in the area. Earlier this week, the southern end of the bridge they crossed was the scene of an intense firefight in which one U.S. soldier was killed and seven others were injured.

"I was a little nervous," admitted Spec. Justin Gates, 21, a radio and telephone operator from Solomons Island, Md. "It's a far cry from my normal duty day."

When the soldiers reached the northern bank, they spread out to different neighborhoods, clearing buildings and occasionally engaging in brief firefights with what commanders described as a mix of Iraqi army troops, Republican Guard forces and paramilitary fighters loyal to President Saddam Hussein's Baath Party.

The troops gathered in teams to invade buildings and houses. During one search, about 10 soldiers entered the yard of a residence. They spotted several Iraqis hiding within a maze of leaves and eventually rounded up eight men who said they lived at the house. Like many of those detained by the Army this week, several of them told troops they were not supporters of Hussein.

"American good," one of the men told the soldiers who tied his hands behind his back and escorted him at gunpoint to a holding pen. "Saddam, no."

"In all, 23 prisoners of war were apprehended during the searches," said Col. Arnold N. Gordon-Bray, commander of the the 82nd Airborne Division's 2nd Brigade.

The searches also uncovered several caches of weapons, including rocket-propelled grenades and AK-47 assault rifles.

Soon after the forces cleared the area near the northern foot of the bridge, Army medics established an aid station in an open-air building. Around noon, a dozen Iraqi civilians -- including three children -- filled the station. Most suffered bullet and shrapnel wounds. An Iraqi surgeon brought several of the civilians into the station and assisted medics.

One man, whose leg was broken and skull was partly exposed by a deep wound, was stitched by the surgeon before being transported by helicopter to another medical facility.

Gordon-Bray said that the division does not plan to hold this city indefinitely but will likely turn things over to other groups, such as humanitarian and nongovernmental agencies, to help civilians rebuild the city to their own wishes.

"We are not going to be an occupying force in any dimension," Swannick said.