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

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

Meanwhile, in southern Iraq, a British soldier was killed and three wounded when their patrol was blasted by a roadside bomb on a road outside the port city of Basra, the defence ministry in London said.

An AFP photographer saw casualties evacuated from a damaged "Snatch Land Rover" -- a lightly armoured version of the army's all-purpose transport -- and flown away in a Lynx helicopter.

In Baghdad, US army Major Steven Lamb said the patrols helped keep Baghdad's militia fighters "in a box."

"It's kind of like having a cop drive down your street a couple times a night, when that happens you have fewer break-ins," he explained.

Apache attack helicopters patrolled overhead and only occasional gunfire or a distant explosion disturbed the calm provided by a weekly vehicle curfew meant to protect worshippers at Friday prayers.

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

US spokesman Lieutenant Colonel Scott Bleichwehl had confirmed operations were underway in the southern Sunni suburb of Dura and the northern Shiite 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.

In a separate setback for the coalition, US helicopter gunships killed five Kurdish policemen -- normally staunch allies of American forces -- during an operation targeting an alleged Al-Qaeda bomb factory.

"Coalition forces killed five armed men during a raid" in the northern city of Mosul aimed at an Al-Qaeda network in the Al-Karama district, the US military said, referring to the Arab Sunni-led insurgent group.

"The armed men were later identified as Kurdish police," the statement said, adding that "coalition forces express deepest sympathies to the families of those individuals killed."

In Karbala, a cleric representing top Shiite cleric Grand Ayatollah Ali al-Sistani called for a "peaceful demonstration" on Monday to mark the first anniversary of a Samarra shrine that triggered the sectarian bloodshed.

Violence raged 80 kilometres (50 miles) south of Baghdad, where police said 11 people were kidnapped and murdered early Friday in the predominantly Sunni village of Mahawil, police said.

Thirteen people were abducted from their home around midnight, Captain Muthanna Hassan said and, while two were released, the others were found floating in the Al-Malih river at dawn.

In addition to those bodies, the bullet-riddled corpse of an employee of the local municipality charged with evicting squatters from state-owned properties was found in Amara, police said.

Three US soldiers also died in the volatile western Sunni province of Al-Anbar on Thursday, bringing the military's losses in Iraq since the March 2003 invasion to 3,106, according to an AFP count based on Pentagon figures.