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The American military was treating with caution a surprise move by Shiite cleric Moqtada al-Sadr that could help pave the way to stability in Iraq.
Sadr said his 160,000-strong Mehdi Army had suspended all armed actions for six months to remove rogue elements from the militia after 52 people were killed in gun battles in the southern Iraqi city of Kerbala during a Shiite festival.
Iraq's national security adviser Mowaffaq Rubbaie welcomed the move, saying it would "contribute to peace and stability".
But a US military spokesman said commanders would be watching to ensure the pledge was followed through.
"What really matters here is actions, and so those are the measures of merit we'll be watching for," said US spokesman Brigadier-General Kevin Bergner.
Asked if the unexpected order meant no attacks on US troops, one senior Sadr aide said: "All kinds of armed actions are to be frozen, without exception."
The battles yesterday appeared to pit Iraq's two biggest Shiite groups against each other - followers of Sadr and his Mehdi Army, and the rival Badr Organisation - the military wing of the Supreme Iraqi Islamic Council - which controls police in much of the south.
Another Sadr aide said: "The aim is to remove bad members who are involved in the Mehdi Army and working for their personal interests ... to hurt the Mehdi Army's reputation."
Analysts said the test of the six-month suspension order would be whether his fighters obeyed because it was no longer clear how much authority Sadr exercised over the Mehdi Army.
It is believed to have fragmented and the US military says rogue factions receive funding, training and weapons from Iran.
Sadr has been high on the US hitlist since leading two uprisings against US forces in 2004.
Washington has in the past called the Mehdi Army the biggest threat to Iraq's security and US officials and Sunni Arab leaders accuse the Mehdi Army of being behind many of the sectarian killings in Iraq.
But Sadr has publicly disavowed violence against fellow Iraqis.
An estimated one million Shiites headed to Kerbala this week for the festival of Shabaniyah, which marks the birth of Mohammed al-Mehdi, the 12th and last Shiite Imam who disappeared in the ninth century. Shiites believe he will return to earth as the redeemer.
Shiite Prime Minister Nouri Maliki said yesterday militants involved in the violence in Kerbala wanted to blow up the Imam Hussein shrine, one of the holiest to Shiite Muslims.
"From our initial investigation, we found some evidence of who did this act ... the intention of this act was to storm into the shrine of Imam Hussein and blow it up," Maliki said from inside the shrine during a visit to Kerbala, 110km south of Baghdad.
Sadr ordered all his movement's offices to close for three days of mourning after the fighting.
Sadr, a fiery anti-American cleric, set up the Mehdi Army in 2003 after the US-led invasion of Iraq. A year later, he led his militia in two uprisings against US forces before getting involved in mainstream politics. He played a key role in Maliki's rise to power last year.
In a split with Maliki, Sadr withdrew his six ministers from the Cabinet in April when the Prime Minister refused to set a timetable for the withdrawal of US troops from Iraq.
Joost Hiltermann, a Middle East analyst with the International Crisis Group think tank, said Sadr wanted to distance himself from what happened in Kerbala.
"What happened yesterday went a bit too far because it involved the holy shrines," Hiltermann said, referring to the violence around the Imam Hussein and Imam Abbas shrines, two of the holiest in Shiite Islam.
"He's showing that he's not in control of these forces. He's embarrassed by it, it's a loose element and he wants to restore control. The question is really whether they will listen."
Earlier, Maliki announced his forces had restored order in the city. The violence among Shiites had spread overnight, with gunmen attacking SIIC offices in at least five cities. Maliki blamed "outlawed armed criminal gangs from the remnants of the buried Saddam regime" for the violence.
The rogue groups in the militia, while proclaiming loyalty to Sadr, have their own agenda. Some are seen as little more than criminals.
Sadr's political bloc holds 30 seats in Parliament and is still part of the ruling Shiite Alliance.
- Reuters