In Ramadi, 100 km west of Baghdad, a two-vehicle convoy was targeted by what the US military described simply as "an explosive device," wounding six soldiers. The seriousness of their injuries was not known.

Residents said that two people on a motorbike had fired a rocket-propelled grenade (RPG) at a US military vehicle, in what seemed to be the same incident.

In a separate shooting on the northern fringes of the capital in the early hours, a US soldier and a six-year-old boy were wounded in a shootout between US forces and a gunman who was with the boy. The gunman was shot dead.

A coalition military spokesman, meanwhile, said an incident in central Baghdad was an RPG attack on a three-vehicle convoy. "One vehicle was damaged," Corporal Tod Pruden said, without confirming reports of casualties.

But a US officer at the scene of the attack on Haifa Street, said that "according to preliminary reports, there are three people wounded."

"It seems now that it was an RPG fired from a vehicle on the street," Major Scott Patton said. "An innocent Iraqi citizen sitting on a street corner was also killed (by the blast), according to reports we are hearing."

He did not comment on witness reports that US troops had opened fire on a car, shooting dead the driver.

Witness Majid Saadi said he believed assailants had thrown a hand grenade at a US armoured vehicle, before setting it on fire with gasoline. An AFP correspondent saw the burned-out wreckage of a military vehicle at the site.

Saadi said he thought the presumed Iraqi driver of the car, which he said had been riddled with bullets, was dead. But it was not clear if the driver was suspected of involvement in the attack.

There was a heavy US presence at the scene, with soldiers closing off the road while they secured the area.

In the shooting incident in the Kadhimyah neighbourhood in the north of Baghdad, US troops were conducting a routine night patrol when they came under fire.

"An Iraqi man attacked the patrol, shooting one of the soldiers.

The soldiers returned fire in self defence, killing the gunman and wounding a boy, who was with the gunman," according to a military statement.

Both of the wounded were evacuated to a military field hospital and were in a stable condition, it added.

In Baquba, 60 km northeast of Baghdad, one Iraqi was killed when a small bomb exploded during a protest to demand the release of local Shiite Muslim leader Ali Abdul Karim Madani, arrested overnight by US forces, witnesses said.

US forces then opened fire in the their direction, wounding four, before breaking up the protest with the help of Iraqi police and tank support, they said.

The attacks come as just the latest strikes on US troops in and around the Iraqi capital, which have increased in recent days, prompting concern that US forces in the country are overstretched.

US forces have suggested that the upsurge may be in reaction to a major sweep it is carrying out north of Baghdad in the hunt for former regime loyalists and Fedayeen militia fighters it blames for the attacks.

That operation, dubbed Desert Sidewinder, had detained a further 32 suspects, according to a military statement.

"The 1st Marine Expeditionary Force received information that the detained individuals were collecting weapons for an attack on coalition forces," it said.

A US Marine was killed and three others were injured while they were clearing a minefield near Karbala, southern Iraq, on Wednesday, another statement said.