Word of the raid came after Iraqi police in Sadr City had said that a bombardment by US helicopters and armoured vehicles had killed nine civilians, including two women, and wounded six others. The police also said 12 people were detained.

The US military said 12 suspects were detained during Wednesday's raids.

"The individuals detained and the terrorists killed during the raid are believed to be members of a cell of a Special Groups terrorist network known for facilitating the transport of weapons and explosively formed penetrators, or EFPs, from Iran to Iraq, as well as bringing militants from Iraq into Iran for terrorist training," the military said.

Meanwhile, Iraqi authorities clamped a three-day driving ban on the capital and erected new checkpoints, while thousands of Shia pilgrims began their annual trek toward a mosque in northern Baghdad to mark the anniversary of the death of one of Shia Islam's key saints.

First-aid tents stocked with coolers of bottled water or offering food, dates, yoghurt and tea lined the streets as authorities scrambled to prevent a catastrophe from marring the ceremonies honouring Imam Moussa ibn Jaafar al-Kadhim, one of 12 principal Shia saints who died in the year 799.

More than a million Shia faithful flogging themselves with iron chains and slicing their foreheads with swords are expected to process Thursday toward the shrine of Imam al-Kadhim in Baghdad's Shia Kazimiyah neighbourhood. The self-flagellation slowly turns their white cloaks red with blood in a ritual of grief banned under Saddam Hussein.

Sunni insurgents often target such gatherings. In 2005 the march was hit by tragedy, when thousands of Shia pilgrims, panicked by rumours of a suicide bomber, broke into a stampede on a bridge. About 1,000 people died.

The top US ground commander in the area, Task Force Justice leader Lt Col Steve Miska, said Wednesday that hundreds of additional Iraqi security forces had been deployed in Kazimiyah, but that American troops would stay away from the shrine out of religious sensitivity.

"There's paranoia surrounding this shrine. If anything happened here, it'd make the Golden Dome look like a precursor," Miska said, referring to the al-Qaeda bombing of Samarra's Askariya shrine, which destroyed the mosque's golden dome and set off a wave of sectarian bloodletting that continues today.

Iraqi Prime Minister Nouri al-Maliki, meanwhile, arrived in Iran on Wednesday for talks expected to focus on bilateral relations and improving the security situation in Iraq. It was the Iraqi premier's second visit to Tehran in less than one year.

"We are here today to boost commercial and security relations with neighbouring countries," al-Maliki told The Associated Press on the plane to Iran. He said he would focus on overcoming "terrorism challenges" in the region.

Iraq, which like Iran is majority Shia, has managed a difficult balancing act between Tehran and Washington since the US-led invasion in 2003, trying to maintain good relations with its powerful neighbour while not angering the Americans.

The US has accused Iran of providing money and weapons to Shia militias in Iraq. Iran denies the charges,

Baghdad residents awoke to find themselves facing a vehicle ban earlier than expected.

The Iraqi military spokesman Brig Gen Qassim al-Moussawi announced a curfew banning all cars, trucks, motorcycles and carts from moving in city streets that had been slated to begin at 10 pm Wednesday would begin 5 am instead. It was to last until 5 am Saturday.

Soldiers were deployed about 100 yards apart on streets in western Baghdad and traffic was barred by barbed wire and warning signs.

Pilgrims wearing traditional white frocks and waving green Shia flags walked from all points of the capital toward the golden-domed mosque where al-Kadhim is believed to be buried in the northern neighbourhood of Kazimiyah. Many men wore wet towels on their heads for relief from the heat.

On a major highway, tents decorated with posters of radical Shia cleric Muqtada al-Sadr provided a shady place to rest.

Iraqi military vehicles played religious songs on loudspeakers. Security forces frisked men and searched women's bags while offering them water.

Talib Madhloom, a retired 53-year-old teacher who was making his way east to the shrine said it was important to honour al-Kadhim despite the heat and security concerns.

"We get bravery, courage and patience from the martyrdom of Imam Moussa al-Kadim," he said. "He was poisoned to death while he was unjustifiably imprisoned for 14 years and he was named for his self control over his anger."

An 11 year-old girl walking in central Baghdad held the hands of her two younger brothers as they walked ahead of their family. She was heading to an Iraqi Red Crescent tent seeking water. "We came early in the morning from Rashid area. We are so tired," she said.

Um Mohammed, 50, had crossed into Baghdad from the volatile province of Diyala to the north.

"All Diyala people came on foot after conducting dawn prayers this morning. The road was packed with walking people from Diyala," she said. "I came with my daughters and daughters-in-law. I could not walk as fast as they did, so I told them to go ahead of me. My knees could not hold me anymore, so I had to sit dawn every now and then."

By Wednesday morning, some 1,500 pilgrims had already passed through one of several checkpoints into the area, according to an Iraqi police lieutenant who identified himself only as Fadil, because of security concerns.

Some 400 plainclothes Iraqi agents, part of a mostly Shia organization called the Public Order Committees, would take up positions at checkpoints, Miska said.

US officials said more than a million pilgrims were expected Thursday in Kazimiyah, but some Iraqi officials put the figure at four million.

While the streets of Baghdad were relatively calm with the driving ban, violence struck Iraqis elsewhere.

A roadside bomb exploded near a police patrol in the northern city of Kirkuk, killing one officer and wounding six others.

Gunmen also targeted the former mayor of the Shia holy city of Najaf who is now an official at a water treatment centre in a drive-by shooting. The official, Tala Hillal, was wounded and his driver was killed in the attack, the latest in a series of assassination attempts against clerics, academics and security officials.