Thousands of U.S. and Iraqi troops set up outposts Sunday in southern parts of Ramadi as part of an operation to establish Iraqi army bases in the country's largest Sunni Arab city and wrest it away from months of insurgent control.

U.S. commanders stressed that the operation was not a large-scale assault on the city but rather an "isolation" tactic to prevent insurgents from receiving supplies or reinforcements from outside. Arab television networks and some Western outlets have reported on an impending attack on the city.

Two long columns of U.S. and Iraqi armored vehicles late Saturday encircled the southern side Ramadi, the capital of volatile Anbar province, and met little resistance.

"The good news is that we didn't get as much resistance as we're prepared for," said Lt. Col. V.J. Tedesco, commander of the 1st Battalion, 37th Armor Regiment, 1st Armored Division. "I really think the fight will be in the coming days."

Large swaths of Ramadi have been in insurgent control for months. Powerful roadside bombings and gunbattles take place every day, confining U.S. patrols to small sections of the city. Prominent tribal leaders who have cooperated with U.S. forces have been assassinated or forced to flee outside the country.

The overnight operation involved thousands of U.S. and Iraqi troops who trickled out of area bases and began erecting two outposts where Iraqi soldiers are expected to begin patrolling a southern neighborhood known as the Second Officer's Quarter.

The area of about 10,000 people, a cluster of homes once set aside for Baath Party officials and Iraqi soldiers in an artillery brigade, has rarely seen U.S. or Iraqi troops.

Resistance was initially light, with a handful of roadside bombs discovered and detonated, but commanders expected insurgents to soon fight back.

Insurgents fired two mortar shells that landed about 500 yards away from where the troops were establishing the outposts. U.S. troops fired back, but no injuries were reported.

Iraqi troops were expected immediately to begin patrolling the area outside their new outposts, following the "clear, hold, build" counterinsurgency strategy that U.S. officials have laid out.

The new outposts will also allow U.S. or Iraqi forces to oversee all major entrances into this city of about 400,000. The new outposts are supposed to block key southern routes where insurgents receive many of their supplies and reinforcements, commanders said.

U.S. commanders said they had reports of residents fleeing before the operation, but that the numbers were much smaller than reported.

"We are seeing some people leaving, but not an exodus," said Col. Sean MacFarland, commander of the 1st Brigade, 1st Armored Division. "The numbers are in the dozens to hundreds (of families), not anywhere approaching a thousand."

It was unclear if any additional residents left the city early Sunday. Commanders said they were trying to address fears that a major attack was impending.

"We are putting the word out that people don't need to be afraid and don't need to leave," said MacFarland. To bolster his point that they were not planning a large-scale invasion, MacFarland added that his brigade now had fewer soldiers than it did earlier in the month.

Countrywatch: Iraq

U.S. commanders said there were equal numbers of Iraqi and American troops in several battalions involved in the operation, but no figures of forces were given.

The operation had a considerable show of a wide array of forces: in addition to long rows of tanks, Marines in boats patrolled the Euphrates River while Navy Seals and Iraqi sniper teams established positions. An Iraqi tank company was also involved in the effort.