MOSUL, Iraq — Iraq's third-largest city looks like Baghdad did a year ago.

U.S. soldiers drive armored Humvees and tanks through a decimated and dusty landscape. Burned-out cars sit on the street corners, and trash and chunks of concrete litter the medians and the gutters. Poor people from the countryside have flooded the city, but the streets and sidewalks are mostly deserted.

U.S. officials say that al Qaida in Iraq and other terrorist groups have a significant presence in the city and that Mosul is a gathering point for foreign fighters coming across the border from nearby Syria .

On Monday, gunmen killed five U.S. soldiers during a firefight after an improvised explosive device attack on their Humvee, and in the past week, 20,000 to 30,000 pounds of explosives in an insurgent weapons cache exploded, killing 60 people and ripping a huge crater in the city. The next day, a suicide bomber killed the police chief at the blast site.

Terrorists aren't Mosul's only problem. The city's Sunni and Shiite Muslim Arabs detest each other, and the Arabs distrust the city's Kurdish, Christian and Turkmen minorities. Although 60 percent of Mosul's population of 1.8 million is Sunni, three-quarters of the provincial government is Kurdish, and the Arabs suspect the Kurds of wanting to take over the city.

"We live in chaos," said Sheik Fawwaz al Jarba , a former member of the Shiite alliance in Iraq's central government. He spoke from Baghdad because Sunni insurgents blew up his house in Mosul .

Islamic extremists have found it easy to blend into this backdrop, said Rear Adm. Gregory Smith , a U.S. military spokesman.

Iraqi Prime Minister Nouri al Maliki has vowed a "decisive" battle against al Qaida in Iraq in Mosul and said he was sending more troops, but in fact they were already on their way. How Iraqi and American forces fare in Mosul will test whether the U.S. counterinsurgency strategy and additional American troops can defeat the insurgents or whether they will keep pushing them around Iraq .

The two main American units in Mosul have been on the job only a short time, and the U.S. soldiers are treading warily. The 3rd Armored Cavalry Regiment from Fort Hood, Texas , arrived in late November, and the attached 1st Battalion, 8th Regiment, from Fort Carson, Colo. , arrived in late December.

U.S. and Iraqi forces are using the formula developed in Baghdad last year— building outposts where there's been no security presence and setting up police stations around the city.

At night, the soldiers move out in their armored Humvees to look for insurgents or weapons caches.

During the day, they "meet and greet" Iraqis in Mosul's neighborhoods. Convoys halt and block the roads. An interpreter and one or two soldiers question shopkeepers about everything from insurgent activity to the water pressure in the local primary school.

The units move quickly, knowing that trouble might arrive soon. Soldiers charged with perimeter security point their rifles down streets, scanning for snipers. Some curious children get close and smile, but most others fix the Americans with hard stares.

One recent Saturday morning, Lt. Michael Smith of Johnstown, Pa. , led part of Charlie Company's 4th Platoon from the 1st Battalion, 8th Infantry Regiment, into north Mosul's al Noor neighborhood.

This wasn't the kind of mission Smith's men had trained for. For months, he and the rest of his battalion studied maps of Baghdad , because that's where they were headed until a little more than a month ago.

During their first day in the city about two weeks ago, part of Smith's platoon was pinned down on a rooftop during a 30-minute firefight. Eight rocket-propelled grenades were fired at them, Smith said. A day later, three platoons lost vehicles to IEDs at a traffic circle.

No soldiers were killed, but at least one must undergo reconstructive face surgery, Smith said.

A few days later, insurgents blew up a school the soldiers had visited.

Smith and his soldiers jumped from the back of their Bradley Fighting Vehicles and banged on the front doors of houses a block from the traffic circle.

There was no electricity and little kerosene, and each home they entered was as cold as the mid-40-degree temperature outside. Women and children quickly retreated to other rooms. The soldiers in their big tan boots tried not to step on the Iraqis' house sandals.

Smith, a West Point graduate, smiled and used the little Arabic he knows to introduce himself. He left the rest to his interpreter.

"We're a new unit. We're new to the neighborhood. We're here to keep you safe," he said.

Some of the Iraqis spoke calmly, but others were visibly nervous and struggled under the constant questions. No one would admit knowing anything about the recent IEDs at the traffic circle, but Smith and his interpreter didn't believe all the answers they were given.

"They're extremely afraid to say anything; I can see where they're coming from," Smith said later. "This is our first step right now."

Outside the last house on the block, a doctor and his adult sons offered little nougat candy bars to the American soldiers.

A nearby explosion interrupted the moment. The Americans crouched and readied their weapons.

"In the house! In the house!" Smith yelled from the door.

Some of the soldiers stormed upstairs to see if they could spot an assailant from the windows.

Someone fired a rocket-propelled grenade at one of the vehicles from near the traffic circle. No one was hurt, and the moment passed.

The challenge for the American soldiers in Mosul , however, remains.

In an interview, Lt. Col. Michael Simmering , the executive officer for the 3rd Armored Cavalry, predicted a short-term rise in violence as coalition forces push into Mosul's neighborhoods.

Counting 20,000 police officers, security and coalition forces total about 45,000 in the province, Simmering said.

In the past month, Simmering, of Harker Heights, Texas , can point to a market in the city that's reopened, a road repaved and a few reopened shops.

The rest of the city, however, remains dangerous.

The surge of U.S. and Iraqi forces has caused some al Qaida members to leave the city, but Jarba, the Shiite sheik, thinks they'll return when the soldiers leave.

"They still have their eyes there, calling them and telling them," he said.

The sheik also cast doubt on Mosul's police force, which coalition forces are counting on to help secure the area. He said the police have been infiltrated by terrorists or are on the payroll, but not in uniform.

Several U.S. Army officers said they think the Iraqi police and soldiers have improved in the last few years.

Jarba thinks an awakening council, or concerned local citizens group, such as those in Anbar province and Baghdad , where the U.S. coalition pays residents and former insurgents $300 a month to protect their neighbors, should be allowed to take root.

The insurgents who blew up Jarba's house killed or threatened many others who tried to form a council or join the police, he said.

Although administration officials in Washington praise the councils for helping to restore order in Anbar province and in Baghdad , Maj. John Oliver of the 3rd Armored Cavalry said he doubts that they'd work in Mosul .

"You don't have that strong tribal structure here," he said. "One tribe couldn't maintain control here."

Still, the stakes are high in Mosul .

"Everyone says Mosul is the exception to the rule," Simmering said. But, "if there were no exceptions to the rule in Iraq . . . how powerful of a message would it be to send to al Qaida and the rest of the terrorists in Iraq ? I think it would finally address what we've been trying to get at for years."

(Lannen reports for the Lexington (Ky.) Herald-Leader . McClatchy special correspondent Hussein Khadim contributed.)