MAHMOUDIYA, Iraq - U.S. soldiers fought exhaustion and braced for the worst yesterday as the military pressed forward with a six-day-old search for three missing comrades believed captured by al-Qaeda in Iraq in an ambush south of Baghdad.

As the search continued, the military announced yesterday that five more U.S. troops were killed and nine wounded in fighting in southern Baghdad and Diyala province.

Even if the three missing troops are dead, soldiers said that the families back home needed to know what happened and that the attackers must be punished.

"We'll find them," Lt. Col. Michael Infanti said. "I'll tell you what: They're going to wish they never did this thing."

Infanti, commander of the Fourth Battalion, 31st Infantry Regiment, gave an impassioned pep talk to soldiers at a patrol base who were exhausted from the search.

"I can't tell you they're alive, and I can't tell you that they're not," he said. "This ain't over. This ain't over by a long shot."

Thousands of troops are involved in the search, backed by aircraft, intelligence agents and dog teams.

Many troops from the Second Brigade, 10th Mountain Division, their faces red from the sun after scouring fields and villages for clues, said hope kept them going.

"It's knowing that there's a chance that they're still alive - and even if they're not alive, their families deserve to know what happened to them," Staff Sgt. Dustin Parchey said at a dusty forward base near Mahmoudiya, 10 miles east of where the ambush occurred last Saturday.

But while Parchey worried about his fellow soldiers, the 29-year-old from Lewistown, in central Pennsylvania, defended the strategy of having remote outposts like the one where the three Americans were captured and four comrades were slain.

"You have to keep a presence there," he said. "The outposts that we have put out there have made a difference."

Soldiers learned yesterday that DNA testing had identified the fourth soldier killed in the Saturday attack. He was Sgt. Anthony J. Schober, 23, of Reno, Nev., according to relatives who spoke to the Reno Gazette Journal. He was on his third tour in Iraq.

Insurgents attacked the post with small-arms fire and hand grenades, breaching the concertina wire that surrounded the two humvees that comprised the stationary outpost. Shell casings found around the two vehicles indicated the soldiers - from the 31st Infantry Regiment, 10th Mountain Division - had put up a fight.

At the base near Mahmoudiya, some soldiers in support roles expressed regret that they were not directly involved in the search but said they were spending sleepless nights doing whatever they could to help.

"I'm doing good, considering," said Pfc. Joel Palmer, 20, of Atlanta.

Palmer, assigned to the Third Infantry Division, is working with the communications network, but he said his division comrades felt the loss as if it were one of their own.

"Every soldier that takes a hit is rough for morale," he said.

He said he believed the missing soldiers were still alive based on information circulating the base.

"More than likely, they've been tortured," he said, "but I'm still saying they're alive. That's all you can say."

Sgt. Jose Atilano of Brosser, Wash., estimated he had gotten 14 hours of sleep since the search began, returning only briefly to the base after spending hours in the field.

He said he was eager to go back as soon as he got the order.

"We are going to keep going until we find them," he said. "If I were in their shoes, I would hope my battle buddies would do the same for me."

Lt. Col. Robert Morschauser, commander of the Second Battalion, Second Brigade, 10th Mountain Division, which works closely with a brigade of Iraqi soldiers, also promised to continue the search. But he acknowledged it might take a different form at some point.

"I'm very proud of the guys," he said, sitting under a wall map of the area. "Will we change the shape of [the search]? We may. But the effort's going to continue."