BAGHDAD - U.S. officials expressed cautious optimism yesterday that three missing American soldiers are still alive even as troops drained canals and questioned children in the search for the troopers feared captured by al Qaeda.

FBI agents and Australian forensic experts have joined the search for the soldiers, who disappeared after an attack south of Baghdad on Saturday in which four GIs and one Iraqi were killed.

Lt. Col. Randy Martin, a U.S. military spokesman, said five days of searches had produced a number of leads that "point to the fact that these men are still alive."

"There are also reports to the contrary, but we have an obligation to follow on every intelligence tip," Martin said.

"There is cautious optimism that in fact these soldiers can be found alive," he added. "That's what we pray for, that's what we hope for."

Col. Michael Kershaw, a brigade commander, said some physical evidence associated with the missing soldiers had been found, raising hopes they were alive. He refused to say more.

While the search was under way yesterday, three American soldiers were killed and another was wounded in a roadside bombing south of Baghdad, the U.S. military reported. The statement provided no more details, and it was unclear if the victims had been part of the search.

Also yesterday, the military reported that Army Sgt. Allen J. Dunckley, 25, of Yardley, Pa., in Bucks County, was killed Monday in Salman Pak by small-arms fire. He was assigned to the 1st Battalion, 15th Infantry Regiment, 3rd Brigade Combat Team, 3rd Infantry Division, out of Fort Stewart, Ga.

An al Qaeda front organization - the Islamic State of Iraq - claimed responsibility for the attack and warned the Americans to call off the search or risk the safety of the captives.

In efforts to find the missing soldiers, Kershaw said troops had drained at least two of the canals that crisscross the Euphrates River flood plain. Armed patrols have walked for hours along the banks of others looking for any sign of the soldiers.

The missing soldiers were attacked while manning an observation post composed of two Humvees surrounded by concertina wire that had been breached, Kershaw said. They were watching for insurgents placing roadside bombs about 800 yards from their patrol base in a rural villa.

The area is inhabited by Sunni Arab clans that had been loyal to Saddam Hussein, and al Qaeda has been active there for several years. The area has been nicknamed the "triangle of death" because of frequent attacks against Shiites traveling between Baghdad and shrine cities to the south.

Also yesterday, mortar rounds hit the U.S. Air Force base at Taji, 12 miles north of Baghdad, destroying one helicopter and damaging nine others, Iraqi police said.

In Baghdad, an explosion rocked the U.S.-controlled Green Zone, a day after a mortar barrage that killed two Iraqis and wounded 10 other people there.

Across the country, at least 58 Iraqis were killed or found dead in bombings, shootings and mortar attacks, police said. They included 42 bullet-riddled bodies of apparent victims of so-called sectarian death squads. *