Five US soldiers have been killed, including four in a single explosion, during combat operations in the south of Baghdad.

The US military said on Sunday that the soldiers were from Task Force Marne, which is working to stop the flow of weapons, and prevent fighting between Shias and Sunnis in the capital.

The soldiers died on Saturday.

Four other soldiers were wounded in the explosion but no other details of the incident were immediately available.

In a separate statement, the military said a fifth soldier had been killed by small arms fire while on foot patrol southeast of Baghdad on Saturday.

The deaths bring to at least 3,689 the number of US troops killed in Iraq since the 2003 US-led invasion of Iraq.

At least 31 US soldiers have been killed so far in August.

Lieutenant-General Raymond Odierno, the US army's second-in-command in Iraq, said there had been a decline in troop deaths after a sharp rise in May and June.

He said deaths were roughly on a par with July, when 80 soldiers were killed.

About 30,000 extra soldiers have been sent to Iraq since February as part of a security crackdown designed to buy time for Baghdad's fractured coalition government to meet a series of political targets set by Washington.

Officials mourned

In related news, hundreds of mourners turned out to bury an assassinated provincial governor and chief of police on Sunday, as Nuri al-Maliki, the Iraqi prime minister ordered an investigation into their killings.

Governor Khalil Jalil Hamza and Khalid Hassan of the southern Iraqi province of Qadisiyah were killed by a roadside bomb on Saturday as they headed back to the provincial capital, Diwaniyah, from the funeral of a tribal sheikh.

Iraqi police and soldiers tightly surrounded the centre of the Shia city of Najaf, where the officials were to be buried a day after their deaths.

Carrying Iraqi flags and posters of Grand Ayatollah Ali al-Sistani, the country's Shia cleric, the mourners set out from the Shia Supreme Islamic Iraqi Council offices joined by a number of officials.

In Baghdad, al-Maliki's office mourned the passing of Hamza and Hassan and announced that an investigation into their deaths was under way.

"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," it said in a statement.

'Cowardly act'

Jalal Talabani, Iraq's president, had described the attack as a "cowardly terrorist act" by Sunnis fighters who had been displaced by the current security crackdown by Iraqi and US forces.

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

The governor belonged to the Shia Supreme Islamic Iraqi Council, formerly known as the Supreme Council for the Islamic Revolution in Iraq, and was a member of al-Maliki's beleaguered coalition government.

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US troops in Iraq (archive)