But more than three-quarters of the senior militant commanders escaped the city. Those who remain, one U.S. officer said, are a "hardline group of fighters who have no intention of leaving."

Baqouba, the capital of volatile and extremely dangerous Diyala province, is less than an hour's drive northeast of Baghdad. U.S. and Iraqi forces are fighting to take back the province — part of a series of offensives targeting militants in districts flanking the capital.

Brig. Gen. Mick Bednarek, assistant commander for operations with the 25th Infantry Division, estimated several hundred al-Qaida fighters remain in the western half of the city.

"They're clearly in hiding, no question about it. But they're a hardline group of fighters who have no intention of leaving, and they want to kill as many coalition and Iraqi security forces as they possibly can," he said in an interview with The Associated Press and another news agency.

"It's 24-7 for us here, and it's probably the same for our adversary as well," he said. "It's house-to-house, block to block, street to street, sewer to sewer — and it's also cars, vans — we're searching every one of them."

U.S. commanders have acknowledged, however, that al-Qaida's sophisticated intelligence gathering meant top militant leaders knew the attack, which began Monday, was imminent.

More than three-quarters of the senior al-Qaida leaders holed up escaped as American soldiers launched an offensive earlier this week, Lt. Gen. Raymond Odierno, the U.S. ground forces commander said on Thursday during a one day trip to the battlefield.

"We believe 80 percent of the upper level (al-Qaida) leaders fled, but we'll find them," Odierno said after meeting with battalion commanders in a bombed-out hospital in downtown Baqouba. "Eighty percent of the lower level leaders are still here."

Soldiers spread maps across rubble and pulled up charred concrete blocks as stools inside the crumbling building. Controlled explosions of roadside bombs boomed in the distance. Soldiers laden down by body armor mopped sweat from their faces.

Days before the offensive, unmanned U.S. drones recorded video of insurgents digging trenches with back-hoes, said Maj. Robbie Parke, spokesman for the 3rd Brigade, 2nd Infantry Division that is doing most of the fighting in western Baqouba.

About 30 roadside bombs — known as improvised explosive devices or IEDs — were planted on Route Coyote, the U.S. code name for a main Baqouba thoroughfare, said Parke, 36, from Rapid City, S.D. "So they knew we were coming."

Odierno, who was in charge of Baqouba as head of the 4th Infantry Division in 2003 and 2004, said he was shocked at how entrenched al-Qaida had become.

"This is not the Baqouba I knew, and we can't let this happen again," he said. Militant activity spiked in Baqouba in the summer of 2006, Odierno said. A U.S. airstrike killed al-Qaida in Iraq leader Abu Musab al-Zarqawi near Baqouba in June 2006, but not before he could turn the city into a major base for his terror network's operations.

Since last fall, the U.S. has kept a single brigade — 3rd Brigade, 1st Cavalry Division — in charge of all of Diyala province. It was enough to conduct sporadic attacks on al-Qaida, but not sufficiently strong to hold the entire province, Odierno said.

He encouraged battalion commanders to come up with a plan to prevent al-Qaida's return, after the major fighting is over. "It's down the road, but it's what you should be thinking about right now," warning "the heavy fighting still might be ahead of you."

Since Monday, two U.S. Army battalions have launched air assaults to the south and west of the area, a tangle of narrow dirt and paved roads crisscrossing a residential area. Troops discovered at least seven homes booby-trapped with trip wires, said Col. Steve Townsend, commander of the 3rd Brigade, 2nd Infantry Division.

Two more units moved in to flank the north and east to block the militants' escape. But by then, Odierno said, many were already gone.

"It's like jelly in a sandwich — it squirts when you squeeze it," Parke said. "We're fooling ourselves if we think we can hold them in."

Four days into the offensive, about 15 percent of western Baqouba has been cleared, and a vehicle ban is in place, Parke said. The entire operation was expected to last 30 to 60 days, he added.