Under the cover of a moonless night, U.S. soldiers on Sunday strung nearly a mile of razor-sharp concertina wire across the northern edge of a neighborhood dominated by insurgents to prevent them from fleeing without a showdown.

Several small teams of five or six troops quickly uncoiled spools of wire and fastened it along the deserted sidewalk of a broad thoroughfare. The cordon was intended to prevent insurgents from blending in with the hundreds of people who fled the city during heavy clashes Sunday.

"The idea is to trap them in Sarai or force them toward our checkpoints to the south," said Col. H.R. McMaster, commander of the Army's 3rd Armored Cavalry Regiment, referring to the neighborhood that U.S. forces believe has served as a launching point for many attacks in the city. "We don't want them to slip out."

More than 5,000 U.S. and Iraqi soldiers entered Tall Afar on Friday in a broad sweep for insurgents who have held sway in the northern city since a previous U.S. invasion, and subsequent withdrawal, last September. In three days of fighting, as many as 200 insurgents have been killed, McMaster said. Two U.S. and four Iraqi soldiers have been wounded, none seriously.

The assault in Tall Afar, considered a transit point and logistics hub for insurgents operating across northern Iraq, is the largest on an Iraqi city since the invasion of Fallujah in November. Commanders say they believe that perhaps a few hundred insurgents remain in the city, but acknowledge that they do not know the exact number. Other than in Fallujah, where entrenched fighters battled advancing U.S. troops, most assaults on Iraqi cities have met little resistance, as insurgents have fled to surrounding areas or blended into the local population.

Soldiers here are hoping to prevent that from happening by converging from all directions on Sarai, the commanders say. Weeks before the invasion, the troops set up checkpoints on the roads to town and built an 80-mile berm around the city to stop vehicles from leaving.

About 500 people attempted to leave the city this weekend through the U.S.-manned checkpoints. At least one man suspected of kidnapping and beheading several residents in recent weeks was apprehended when he tried to leave the city with a group of children, McMaster said. When soldiers interviewed the children, they said they did not know the man but went with him because they had been threatened.

A cacophony of gunfire and explosions filled the air around Tall Afar on Sunday, the heaviest day of fighting since the invasion began. Soldiers continued methodically searching homes and questioning residents, frequently coming under small-arms fire that whistled overhead as they passed from house to house or leapt across gaps between rooftops.

Much of the shooting came from a series of deep, grooved valleys that divide the city. The valleys, dipping as low as 40 feet below street level, allow insurgents to stash weapons and move undetected. Soldiers have found large caches of explosives in the valleys and begun firing warning shots when they spot men of military age descending into them.

"With the geography and the buildings so close together, it's a complex terrain to fight in," said Capt. Noah Hanners, 26, of Chillicothe, Ohio, commander of the 3rd Armored Cavalry Regiment's Blue Platoon, Eagle Troop, 2nd Squadron, which cleared several homes along the edge of a valley northeast of downtown Sunday.

Just after 9 a.m., Hanners's platoon used a sledgehammer to knock down the door of a two-story house made of large stone blocks. A woman wearing a purple abaya, or full-length cloak, and holding a baby ran toward them. "Already my husband is dead and you are breaking my house!" she shouted.

When shots rang out in the streets, she clutched her baby close to her chest. As a series of explosions grew closer, she sat down, rocked her baby and began to cry.

"Um, that's close. Very close," said Pfc. Hector Rodriguez, 20, of Buffalo, after an explosion rattled the windows.

Soldiers scrambled to set up shooting positions on the roof.

"We are receiving sniper fire," a voice over the platoon's radio said.

"Where from?" someone answered. No one knew. The platoon's machine gunner began shooting staccato bursts into the valley from the next roof.

"Who is that shooting?" Hanners asked over the radio.

"Blue saw team, we saw muzzle flashes from the south," came the response.

Another call over the radio said a Tall Afar resident working with the Army had identified an arms dealer in a house across the street. The soldiers gave the woman with the baby a voucher for $300 to repair her gate, redeemable at a nearby military base, and dashed next door.

By the time they reached that house, the suspected arms dealer and two other men had already been handcuffed and blindfolded by other members of the platoon.

"This is my nephew. He does not even live here," a woman in the house said as the men were led away.

"He was identified. That's what you get for selling weapons," a soldier said.

The soldiers continued to the last house on the block. They herded a family of nine into a living room and searched their belongings. A drawer of silverware crashed to the floor, startling the children.

"I am scared. We are all scared, but what are we supposed to do?" said the father, Shimsdain Muhammed, 40, an electrician, according to an account of his remarks provided by a military interpreter traveling with the unit. Muhammed told his children to wave and smile at the soldiers, who responded in kind. "I cannot tell them to leave, so we will do the best we can."

Suddenly, an explosion outside shattered the windows of the house, eliciting screams and confusion. It turned out to be a passing U.S. M1-A1 Abrams tank firing its main gun into the valley.

"This place is too much," another soldier said later, recounting the story to a friend.

Elsewhere Sunday in Tall Afar, an Iraqi army unit freed 35 hostages held in a house south of downtown, according to Maj. Gen. Khorsheed Salim, commander of the army's 3rd Division, which is heavily involved in the operation.

Soldiers in the western part of the city found a laboratory rigged with explosives, McMaster said. The lab also contained a chemical that burned the troops' throats and eyes when they entered. The Army is trying to identify the substance.

[Early Monday, as many as 30 gunmen in 10 cars opened fire on Iraq's Interior Ministry, killing two policemen and wounding five, the Reuters news agency reported.

An Interior Ministry official said gunmen used rocket-propelled grenades, mortars and automatic rifles in the attack.]

Soldiers from the Army's Blue Platoon, Eagle Troop, 2nd Squadron, 3rd Armored Cavalry Regiment rest during house-to-house searches in the city of Tall Afar.