The Americans were killed on Saturday and all belonged to Task Force Marne, which was deployed in the southern belt of the Iraqi capital four months ago as part of the new US counter-insurgency troop "surge".

An explosion killed four of the soldiers and wounded another four during fighting south of Baghdad. The fifth was killed by small arms fire while on foot patrol southeast of the capital, the military said.

At least 29 American soldiers have now died in Iraq in August, and the latest deaths took US losses in the country since the 2003 invasion to 3,684, according to an AFP count based on Pentagon figures.

Sergeant First Class Craig Zentkovich said Task Force Marne deployed to what was an insurgent safe haven outside Baghdad on April 1, part of the massive thrust billed as a last-ditch plan to stablise Iraq after four years of war.

"Their mission is primarily to stop the flow of insurgents and weapons into Baghdad," said Zentkovich.

Some 85,000 Iraqi and US troops have deployed in Baghdad since February to flush out Sunni extremists and Shia militias, but the number of civilians killed has remained high and rose last month to pre-surge levels.

In one of the most high-profile assassinations in months, a bomb attack killed the governor and police chief for the southern mainly Shia province of Qadisiyah on Saturday as they were coming home from a funeral.

Governor Khalil Jalil Hamza belonged to the Supreme Iraqi Islamic Council party, the second largest Shia party in parliament and part of the ruling coalition, while police chief Khalid Hassan was a political independent.

"We have issued orders for an investigation into this criminal act and for those who carried out this cruel crime to be detained so that justice can be done," the office of Prime Minister Nuri al-Maliki said in a statement.

Hundreds of mourners attended the joint funeral of the governor and police chief held under tight security, walking behind their cortege in the holy Shia city of Najaf to hear prayers at the revered Imam Ali mausoleum.

Carrying Iraqi flags and posters of revered Shia cleric Grand Ayatollah Ali al-Sistani, the mourners set out from the local Shia Supreme Islamic Iraqi Council office, joined by governorate and political officials.

Iraqi police and soldiers tightly guarded Najaf's old city around the shrine and vast cemetery, considered a sacred burial ground for millions of Shias and where the governor and police chief were to be laid to rest.

In Diwaniyah, the provincial capital of Qadisiyah, deputy governor Diaa Abdulkarim, lifted a curfew ordered after the killings and confirmed that while an investigation was ongoing, it had not yielded any results so far.

In Baghdad, President Jalal Talabani blamed extremists flushed out of notorious Sunni flashpoints by Iraqi and US forces continuing a six-month-old security crackdown for what he labelled a "cowardly terrorist act."

"They have committed a crime in a secure part of our country after they were besieged and kicked out of Anbar, Diyala and Samarra," his office said.

But Diwaniyah is also a target for rival Shia factions battling for supremacy in the region, and the governor was a member of the Badr Organisation, the armed wing of the Supreme Islamic Iraqi Council.

Six policemen in the Shia-dominated police force, a soldier and a woman, were killed in random attacks across northern Iraq, local commanders said.

In the deadliest single attack, gunmen ambushed a police patrol rumbling down the main street in the village of Arab Koy, south of the troubled northern oil city of Kirkuk, killing three policemen, said Major Hassan Jassim.

The woman was killed when a roadside bomb exploded in the northern city of Mosul, said Colonel Ali Ahmed, and the soldier shot dead in an ambush in the town of Hawijah, police Captain Farhad Abdullah said.

The US military said it arrested 30 suspects in raids targeting al-Qaeda and bombing networks in western Baghdad and north of the capital.