The aide, Sheik Hazim al-Araji, said on Iraqi state television that the goal was to "rehabilitate" the organisation, which has reportedly broken into factions, some of which the United States maintains are trained and supplied by Iran.

"We declare the freezing of the Mahdi Army without exception in order to rehabilitate it in a way that will safeguard its ideological image within a maximum period of six months starting from the day this statement is issued," Sheik al-Araji said, reading from a statement by Muqtada al-Sadr.

In Najaf, Muqtada al-Sadr's spokesman said the order meant the Mahdi Army would no longer launch attacks against US and other coalition forces.

"It also includes suspending the taking up of arms against occupiers as well as others," Ahmed al-Shaibani told reporters. Asked if Mahdi militiamen would defend themselves against provocations, he replied: "We will deal with it when it happens."

The order was issued following two days of bloody clashes in the Shi'ite holy city of Karbala which claimed more than 50 lives. Iraqi security officials blamed Mahdi militiamen for attacking mosque guards, some of whom are linked to the rival Badr Brigade militia.

A spokesman for Muqtada al-Sadr, Ahmed al-Shaibani, denied that the Mahdi Army was involved in the Karbala fighting.

Tensions have been rising in southern Iraq as rival Shiite groups manoeuvre for power, especially in the oil-rich area around Basra, Iraq's second-largest city.

Muqtada Al-Sadr organised the Mahdi Army shortly after the US-led invasion of 2003. Since then the Mahdi Army has become the most active and feared armed Shi'ite group, blamed by the US for driving thousands of Sunni Muslims from their homes in retaliation for Sunni extremist attacks on Shi'ite civilians.

The Mahdi Army launched two major uprisings against US and coalition forces in 2004. Since then, the Americans have differentiated between the mainstream Sadrist organisation and what they term "rogue" elements within the force that have staged numerous deadly attacks against U.S. forces in Baghdad and elsewhere.

Meanwhile a US military jury in Fort Meade, Maryland, yesterday sentenced the only officer court-martialled in the Abu Ghraib abuse scandal to a reprimand, sparing him any time in prison for disobeying an order to keep silent about the prison abuse investigation.

The jury had acquitted Army Lt Col Steven Jordan of all three charges directly related to the mistreatment of detainees at the US-run prison in Iraq.

Photographs showed US soldiers grinning alongside detainees held in humiliating positions.

Jordan never appeared in any of the photos, but as director of the prison's interrogation centre and the highest ranking officer there at the time, he had been accused of fostering a climate conducive to abuse.

Jordan was convicted of a single charge: disobeying a general's order not to discuss the abuse investigation.

The reprimand was among the lightest sentences Jordan could have received.