BAGHDAD — US soldiers raided suspected safe-houses near the Euphrates River south of Baghdad yesterday in their search for three comrades captured by insurgents linked to al-Qaida but found them empty after the militants apparently were tipped off and fled, an official said.It was the latest in a series of frustrations for exhausted US troops hunting for any sign of the troops who went missing after a co-ordinated May 12 ambush that left four other Americans and an Iraqi soldier dead.

Maj. Webster Wright, a spokesman for the 10th Mountain Division’s 2nd Brigade Combat Team, also said some 27 air assaults had been staged in the last nine days, dropping off soldiers to gain the element of surprise and to avoid the bomb-studded roads.

That was four times the entire number staged by the division that previously controlled the area, he said.

“It’s very quick it’s very sudden,” he said. “You can come from any different angle and drop down on a house without being pinned to the roads.”

But he said the insurgents were catching on to US tactics and had fled ahead of raids of a couple suspected safe-houses near the river yesterday.

“We went after a couple of objectives and they had some warning systems out,” he said. “They were able to slip away from us, but we’re going to come at things from a different angle.”

He said it was looking at targets to the west of the division’s area of operations. He wasn’t more specific, but that would likely include the volatile Anbar province, a vast desert area that stretches to the borders of Syria, Jordan and Saudi Arabia.

“We are looking at all of the possibilities that the enemy might be taking.

Some of those might options might be to the west,” he said.

Meanwhile, the platoon that was decimated by the attack on a stationary observation outpost near the village of Qarghouli began to regroup, receiving eight replacements for those who were killed or captured.

The platoon leader, Lt. Morgan J. Spring-Glace, 25, of Worcester, Massachusetts, said he decided to take the reinforcements on their first operation yesterday after failing in a last-ditch attempt to find the perpetrators of the attack with just the old crew the night before.

“We’ve had two missions in the past 12 hours, and this last one my new guys went on, and we just got them like 12 hours ago. I don’t even know all of their names. It’s been a hectic schedule for everyone. Regardless of the time, they’ll come back, they’ll get a few hours rest and go back out,” he said.

“We went out on a mission last night, I didn’t take these new guys. We didn’t want to take them because we didn’t need to take them. It was kind of like closure. We wanted to catch someone last night. and it would have just been something that we could have done,” he added.

The soldiers from the Company D, 4th Battalion, 31st Infantry Regiment — nicknamed the “Polar Bears” — fought exhaustion and dehydration as they combed the fields and searched houses in temperatures soaring above 38 degrees Celsius (100 degrees Fahrenheit). Even the search dogs were affected by the heat.

During one house search, the soldiers were instructed to comb through everything, looking for undershirts and pieces of uniforms, anything that the missing soldiers could have been wearing that night.

“They’ve been keeping busy. These guys are walking themselves to death. These guys are almost in some cases getting run into the ground,” Spring-Glace said.

The troops also are facing multiple dangers as they navigate the difficult terrain in the Sunni insurgent stronghold known as the triangle of death, ranging from firefights, snipers and bombs planted in the fields and left on the roadside.

Maj. Gen. Rick Lynch, commander of US troops south of Baghdad, told CNN in an interview that some 37 company-level operations are taking place, with 4,000 US soldiers and 2,000 Iraqis involved in the search effort.

He expressed cautious optimism that the three missing troops — Pfc. Joseph J. Anzack Jr., 20, of Torrance, California, Spc. Alex R. Jimenez, 25, of Lawrence, Massachusetts or Pvt. Byron W. Fouty, 19, of Waterford, Michigan — were still alive.

“We won’t stop until we find our fallen comrades,” he said, adding that the military is following about 249 intelligence reports, the majority of which say the three soldiers are still alive. “We’re pursuing all leads with a passion, but right now we believe our soldiers are still alive, each day that passes when we don’t see proof of life, it causes us concern.”

Lynch also said more than 1,000 people had been detained for questioning, two of whom had confessed to being part of the May 12 attack. “So they’re giving us actual intelligence and we continue to follow through,” he said.