For the fourth time this year a bomb exploded in bustling Tehran Square in downtown Baghdad, wounding at least 20 day labourers waiting at a spot popular for seeking work, security and medical officials said.

In the north of the city, a bomb exploded in a mixed Sunni and Shiite district, killing two civilians, according to a security source

And in Samawa, south of the capital, gunmen murdered two women and a girl from the same Shiite family, in an apparent sectarian attack, police said.

The latest attacks came after a US military spokesman told reporters that attacks by car bombs and roadside booby traps are running at an all-time high, three-and-a-half years after US-led forces overthrew Saddam Hussein.

US casualties are also mounting. Four more soldiers were killed on Wednesday in southwest Baghdad when their unit was attacked by gunfire and mortars.

Since Monday, 14 US soldiers have been killed, mostly in Baghdad, in a spike in casualties that brought the number killed since March 2003 to 2,729, according to an AFP count based on Pentagon figures.

Iraqi and US forces have responded to the violence wracking Baghdad with Operation Together Forward, a joint security plan which has brough 15,000 US troops and more than 40,000 Iraqi soldiers and police onto the streets.

By Monday, they had "cleared approximately 95,000 buildings, 80 mosques and 60 muhallas (small administrative districts), detained more than 125 terrorist suspects, seized more than 1,700 weapons, registered more than 750 weapons and found 35 weapons caches."

"The combined forces have also removed more than 196,921 cubic meters (seven million cubic feet) of trash from the streets of Baghdad," a US statement said.

House-to-house searches have been cut back during the Mulsim holy month of Ramadan, however, and Prime Minister Nuri al-Maliki has been reluctant to allow US troops to raid Sadr City, stronghold of the Shiite Mahdi Army militia.

US intelligence officers believe that Mahdi militiamen, who are nominally loyal to the radical anti-American cleric Moqtada al-Sadr, are linked to the death squads that hunt political and sectarian targets on Baghdad's streets.

Sadr is, however, a powerful figure and his supporters are a key component of Maliki's fragile coalition government.

The loyalty of Iraqi security forces has also been in question, with some Shiite-dominated units accused of collaborating with the militias and death squads fighting a sectarian dirty war which leaves 100 people dead every day.

Iraq on Wednesday demobilised an entire 800-strong police brigade and quarantined them in a US military base where they will receive what a US spokesman said was "anti-militia, anti-sectarian, national unity training".

"There was clear evidence that there was some complicity in allowing death squad elements to move freely, when in fact they were supposed to be impeding their movement," Major General William Caldwell told reporters.

The development of Iraqi security forces is of vital important to President George W. Bush's administration, which has pinned its strategy on training local troops to replace the 142,000 US soldiers in the country.

But the discipline of US forces has also been called into question.

On Wednesday, two US Marines pleaded not guilty to murdering an Iraqi civilian, at a pre-court martial hearing in Camp Pendleton, California.

The charges relate to the death of Hashim Ibrahim Awad on April 26 in Hamdania. Prosecutors allege that a gang of eight US servicemen kidnapped the 52-year-old from his home, shot him dead and planted a rifle near his body.