Thousands of US 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 begin wresting it away from months of insurgent control. US commanders stressed that the operation was not a large-scale assault on the city - Arab television networks and some Western outlets have reported on an impending attack on the city - but rather an "isolation" tactic to prevent insurgents from receiving supplies or reinforcements from outside. Large swathes of Ramadi have been in insurgent control for months. Powerful roadside bombings and running gunbattles in the street take place everyday, confining US patrols to small sections of the city. Prominent tribal leaders who have cooperated with US forces have been assassinated or forced to flee outside the country.