Witnesses said US and Iraqi troops had cordoned off areas in frontline districts in northern and southern Baghdad and were hunting for militants.

"We are still flowing forces into the city," US Lieutenant Colonel Christopher Garver told AFP, while both military and civilian officials warned reporters not to expect instant results from the operation.

The US military has begun moving the first of 21,500 reinforcements into Baghdad in the latest in a series of operations to regain control of a city plagued by insurgent gangs and sectarian death squads.

US Lieutenant Colonel Scott Bleichwehl described the activity as "framework operations" that involved "presence patrols in partnership with Iraqi national police."

Apache attack helicopters circled over the town centre and the sound of gunfire and a distant explosion disturbed the calm resulting from the weekly vehicle curfew imposed to protect worshippers at Friday prayers.

Four Iraqi national police patrol boats sped up the river Tigris, which roughly divides Baghdad between a mainly Shia east and a west viciously disputed by rival Sunni and Shia factions.

Earlier Friday, Bleichwehl had confirmed operations were underway in the southern Sunni suburb of Dura and the northern Shia district of Kadhimiyah, both scenes of recent violence.

"It's a mosaic of activity out there, there are multiple missions," he said.

Bleichwehl and other officials stressed, however, that the plan to deploy a total of 80,000 Iraqi and US troops, is "a rolling, ongoing operation" that will be more intense at some times than at others.

"We are months away right now from seeing any concrete results. We have to stress there is not going to be any magic bullet," US embassy spokesman Lou Fintor told reporters.