BAGHDAD - An Iraqi soldier was watching over the concrete wall on Monday when a .50-caliber round ripped into his head.

Soon after the attack was reported on the tactical radio, two American military advisers were on their way to the scene, laser range finder in hand, to call in a Hellfire missile strike on a sniper position on the far side of a desolate no man's land.

This is the war over the wall. It is a daily battle of attrition waged over the large concrete barrier that the Americans have been building across Sadr City in the hope of establishing a safe zone in the southern tier of the Shiite enclave.

The formal truce that was announced in the Green Zone with great fanfare on Monday has meant nothing here. Shiite militias have been trying to blast gaps in the wall, firing at the American troops who are completing it and maneuvering to pick off the Iraqi soldiers who have been charged with keeping an eye on the partition.

American forces have answered with tank rounds, helicopter rocket strikes and even satellite-guided bombs to try to silence the militia fire. On some stretches, the urban landscape has been transformed as the Americans have leveled buildings militia fighters have used as perches to mount their attacks.

"The enemy kept coming back to some of the same buildings," Col. John Hort, the commander of the Third Brigade Combat Team, Fourth Infantry Division, said during a recent visit at Thawra II, a joint American and Iraqi outpost that abuts a section of the wall that has been a hotbed of militia resistance. "We ended up having to use some larger ordnance out of our Air Force to reduce some of the buildings around here."

Even while American forces deploy reconnaissance drones and satellite-guided rockets, the American strategy in Sadr City is a throwback to a more primitive form of warfare. It depends on concrete - lots of it, which comes in large slabs that are being assembled into an imposing barrier three miles long.

The Americans began building the wall a month ago, working east to west. The work started at night but soon extended into the day as American commanders sought to speed up the construction.

Supporters of Moktada al-Sadr, the anti-American cleric, denounced the wall as a nefarious effort to divide the city. Militia fighters with rocket-propelled grenades, machine guns and small arms have been trying to halt its construction.

Those efforts have failed, and the barrier is now 80 percent complete. But the fighters have blown a few gaps in the wall and, in one instance, appear to have hitched a truck to a damaged slab to yank it down. To make it hard for the Americans to fix the holes, the fighters have continued to seed the strip south of the barrier with explosively formed penetrators, a particularly lethal type of roadside bomb. Some have been hidden in the cracks or depressions in the wall itself.

Lt. Col. Michael Pemrick, the deputy commander of the Third Brigade Combat Team, and his security detail headed to Sadr City in armored MRAP, or mine-resistant ambush-protected, vehicles on Monday, when the cease-fire was announced. Iraqi soldiers have a tendency to defend static positions, and the colonel wanted to see what the Iraqi soldiers were doing to monitor the wall.

Driving to an Iraqi Army position near the barrier, Colonel Pemrick pressed First Lt. Salwan Abd al-Amer to take some of his soldiers and join the Americans on a patrol through the alleys and streets.

A man in a robe known as a dishdasha, who seemed pleased that a security force was finally passing through his otherwise unprotected neighborhood, volunteered that militia fighters had hidden a bomb near a local power generator.

An Iraqi soldier entered the compound and emerged triumphantly with a bag full of explosives. But the patrol also identified a problem: the militias had destroyed one of the slabs in the wall. Though it was just a few blocks from the Iraqi soldiers' position, the opening was undefended.

The explosives were loaded into an Iraqi truck, which the Americans led to an Iraqi battalion headquarters in the rear so it could be picked up by an American explosive ordnance detachment that day.

No sooner had the Americans arrived at the headquarters than there was a radio report that an Iraqi soldier at another combat outpost near the wall had been grievously wounded. Capt. Beau Cleland and Master Sgt. Fernando Alicea, who were on the American team advising the Iraqi battalion, were anxious to get to the scene to call in a helicopter strike. A sniper with a .50-caliber rifle had already shot several soldiers in the battalion and it looked as if he had struck again.

Colonel Pemrick had his security team take the advisers to the front, which was just several blocks away. As the vehicles moved north, the number of civilians on the street dwindled.

With a sniper on the prowl with a large-caliber rifle, the Americans were taking no chances. Climbing out of the armored vehicles, they darted up the street to the charred building that served as an Iraqi Army outpost and raced upstairs.

An Iraqi soldier taking cover in a hall on the second floor urged the Americans to stay away from the windows and to keep low.

"Sniper," he said in English.

The floor was covered with broken concrete, empty cigarette packages and a dusty brown teddy bear, a fleeting reminder that the fighting position had once been someone's home. A splotch of red blood was in a corner room where the Iraqi soldier had been hit. His fellow soldiers said that he had been sitting in a plastic chair and looking over the wall when a round hit the side of his face, ripping across his eyes.

Peering over the bottom edge of window frame, Captain Cleland determined that the shot had probably come from a hole for an air-conditioning unit in the top floor of a three-story structure on the northwest side of the barrier.

The captain used the range finder to determine the precise coordinates, 150 yards to the north. Before he became an adviser to the Iraqi forces, Captain Cleland was a field artilleryman.

"So we are good at this," he explained matter-of-factly.

Sergeant Alicea radioed the coordinates and described the target to a team of Apache attack helicopters. It seemed possible that the sniper had moved with the arrival of the Americans, but another rifle shot rang out.

Sergeant Alicea said it was important to strike nonetheless. Often, a sniper will leave his weapon so he can blend in with civilians on the street, he said. If nothing else, he said, the Apaches would eliminate another hiding place.

"We keep cutting down positions they can use," he said.

After a long wait, a Hellfire missile soared over the soldiers' position and slammed into the building, demolishing a wall on the top floor and setting it aflame.

No one could exclude the possibility that the sniper had moved to a lower floor or was hiding in a nearby building to fire at the Americans as they left the Iraqi outpost. So Staff Sgt. Jason Condreay, the head of Colonel Pemrick's security detail, told the Iraqis his soldiers would unleash a volley of fire toward the ruined building and tossed smoke grenades as the Americans left.

"I am going to throw everything we got," he said. "I am going to lay down suppressive fire and pop smoke."

There was a deafening burst of .50-caliber machine-gun fire as the Americans scampered back to the armored vehicles and climbed inside.

On Tuesday, Colonel Pemrick went forward again to visit an Iraqi outpost near the wall that was commanded by First Lt. Adel Ali.

A small fleet of American armored vehicles was positioned nearby. They were working to extend the wall under the protection of two M1 tanks. The troops came under fire, and an M1 tank answered by firing three rounds into a building just north of the barrier, setting it alight.

As the colonel and his soldiers returned to their base the tactical radio crackled with reports of roadside bombs found, tips about hidden arms caches and enemy fire received.

The Iraqi troops where the soldier had been struck in the head on Monday were holding their ground, but they were under attack again. It was Day 2 of the formal cease-fire, and the fighting was still on.

© 2008 The New York Times