BAGHDAD - Nearly three months after the U.S. military launched a new strategy to safeguard Baghdad's population by pushing American and Iraqi forces deeper into the city's neighborhoods, defending the small outposts is increasingly requiring heavy bulwarks reminiscent of the fortresslike bases that the U.S. troops left behind.

To guard against bombs, mortar fire and other threats, U.S. commanders are adding fortifications to the outposts, setting them farther back from traffic and arming them with antitank weapons capable of stopping suicide bombers driving armored vehicles. U.S. troops maintain the advantage of living in the neighborhoods they are asked to protect, but the need to safeguard themselves means more walls between them and civilians.

At a moonlit outpost on the edge of Baghdad's Sadr City, First Sgt. Donald Knapp balanced himself on a concrete barrier suspended by a crane and slowly guided the heavy slab into position. It was 3 a.m., and Knapp and a few other soldiers were working through the night to fortify their camp.

Over four days, the soldiers erected hundreds of sections of wall and reinforced them with barbed wire and 300 truckloads of sand. They pushed out the walls of the camp, known as a joint security station, and blocked approaching roads with serpentine barriers.

"When the guys get time to sit down, they sleep," said Knapp, a sniper from Milan, Ind., as soldiers in dusty T-shirts labored nearby.

Knapp's unit, from the 82d Airborne Division, is redoubling security efforts as insurgents and militias step up attacks on their outpost, one of the dozens of small patrol bases set up as part of the deployment of tens of thousands of additional U.S. and Iraqi troops.

The strategy has reduced sectarian killings but has put U.S. troops at greater risk; on April 13, a suicide bombing at an outpost east of Baghdad killed nine U.S. soldiers and wounded 20.

Army Gen. David H. Petraeus, the U.S. commander in Iraq, has acknowledged that U.S. troops will face opposition as they move into neighborhoods.

"It's a fight. They're fighting to hold a grip on the population, and the Iraqis and coalition are working to break the grip," he said in an interview last month. Still, he said, the outposts are vital to his counterinsurgency strategy.

"If you want to protect the population, you've got to live with it," he said. "There's no commuting to the fight."

More than 60 joint security stations, staffed by Americans and Iraqis, and U.S. combat outposts are operating in Baghdad, leading to an increase in the discovery of weapons caches, said a U.S. military spokesman, Maj. Gen. William B. Caldwell.

"There's two threats to the combat outpost . . . a huge truck bomb and indirect fire," said Lt. Gen. Raymond T. Odierno, who handles day-to-day military operations in Iraq, in an interview at his Baghdad headquarters. In response, he said, U.S. troops are building more walls to shield themselves from mortars and rockets, while trying to track down insurgents firing on them.

To counter truck bombs, military engineers are gauging the structural soundness of the outposts and making sure they are well removed from traffic, Odierno said. Antitank weapons such as the bazookalike AT-4 are also required for soldiers on guard.

"They are now armoring these trucks, so whereas before we could shoot them and kill them, now we have to use some antitank capability against them, and we're going to do that," Odierno said.

The constant need for vigilance - coupled with hardship conditions and the prospect of 15-month tours - has in some cases taken a toll on morale. Some U.S. soldiers, particularly junior officers, said they accepted the risks to live closer to the Iraqi people. Others said they longed for a sense of purpose and voiced frustration at the prospect of harsh, dangerous tours for a mission they consider murky.

"What do you want us to accomplish over here? We aren't hearing any end state. We aren't hearing it from the president, from the defense secretary," Sgt. First Class Michael Eaglin said in a room cluttered with bunk beds, rucksacks and weapons at the Sadr City outpost.

"We're working hard, and the politicians are arguing. They don't have bullets flying over their heads. They aren't on the front lines, and their buddies aren't dying," Eaglin said, echoing the sentiments of a group of soldiers around him.

"It's almost like the Vietnam War. We don't know where we're going," Spec. Adam Hamilton agreed.

"We're not complaining," Eaglin said. "We're tired of being lost. Have you ever been lost and at the same time getting shot at? It's miserable. . . . I want to be here for a reason, not just a show of force."