Meanwhile, US forces killed three insurgents in raids in western Iraq in which a senior al-Qaeda leader involved in supplying weapons for attacks and preparing propaganda videos was captured, the military said.

A Samawa police captain, speaking on condition of anonymity, said fighting had broken out on Thursday when police attempted to crack down on armed members of radical cleric Moqtada al-Sadr's Mahdi Army militia.

Fighting was continuing, he said.

"The militias planted bombs on the main roads around the Sadr office in the city using large quantities of explosives to prevent the police advance," the captain said, adding police posts had been bombarded with mortars.

Hillal al-Hatchami, an emergency medic at the town's Muthanna Hospital, said at least one policeman and four civilians had been killed in the fighting and 26 people wounded.

Muthanna province governor Mohammed Ali al-Hassani told reporters: "We will not negotiate with the militias as they are outlaws. We will work to impose law and order in the city."

Meanwhile, further north near the town of Hilla, a roadside bomb hit an Iraqi army patrol on the main route east of town on Friday and killed four soldiers, according to Lieutenant Haider al-Husseini.

Four more members of the security forces, police commandos, were killed in a similar attack near the ruined Al-Askari shrine in the northern city of Samarra, a local security official told AFP.

The golden domed shrine is a revered Shia pilgrimage sight and its destruction by Sunni bombers last year was one of the triggers of Iraq's sectarian conflict.

At least four more Iraqis were killed in armed attacks in the north of the country, according to security officials, including Captain Hilkurd Mohammed of the Iraqi army who was gunned down in the oil city of Kirkuk.

Iraq is in the grip of several overlapping civil conflicts between rival political and sectarian factions, and the US-backed government and its American allies face daily attacks by Shia militias and Sunni insurgents.

In Baghdad, what appeared to be a salvo of at least five mortar shells crashed into the fortified Green Zone in an area near Prime Minister Nuri al-Maliki's office and the US embassy.

Five explosions rocked the area at around 2:00 pm (1000 GMT) during the capital's weekly vehicle curfew, which was declared to protect worshippers attending mosques for Friday prayers.

It was not immediately clear if anyone was hurt in the blasts.

The Green Zone, a walled-in city district that protects the US and British embassies and several official Iraqi buildings, is a frequent target of mortar and rocket attacks launched by the city's warring factions.

US commanders have accused Iranian agents of supplying weapons to the groups and inciting them to fire on American targets, a charge denied by Tehran.

Iraq has executed an al-Qaeda fighter who carried out one of the first and deadliest bombings in Iraq's bloody sectarian war, an official said Friday.

"Oras Mohammed Abdul-Aziz, also known as Abu Omar Al-Kurdi from Mosul, was executed on July 3 at 11:00 am," justice ministry undersecretary Busho Ibrahim told AFP. "He was executed by hanging in Baghdad."

On August 2003, senior Shia cleric Ayatollah Mohammad Baqer al-Hakim and at least 84 more people were killed when a car bomb exploded outside the Imam Ali mausoleum in Najaf, the holiest shrine in Shia Islam.

The blast was quickly blamed on Sunni extremists and outraged Iraqi Shias, helping set in motion Iraq's subsequent descent into civil strife.

Ibrahim said Abdul-Aziz had been convicted of his involvement in the blast last year by Iraq's secretive Central Criminal Court and that his death sentence had been confirmed by the Iraqi presidency.

"He was arrested in 2005 -- I believe by the Americans -- then his case was transferred to the Central Court. He was sentenced to death in 2006 upon a presidential decree," he said.

This was the first confirmation that a trial had taken place, although Iraqi authorities did announce an arrest in the investigation in March 2005, then identifying the suspect only as a Kurd from Mosul.