The US military said: "Criminal activities by these individuals propagated instability within Iraq and their removal from the social structure is a critical start to providing the Iraqi populace with a safe and stable environment."
"One cannot believe that the Shia-Sunni civil war was not pre-planned by the West"
Mohammad Karim, Melbourne, Australia
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Combined Iraqi and US forces have carried out 52 operations in the past 45 days focused on the Mahdi Army and 42 operations aimed at Sunni fighters, the statement said.
The recent detentions are in addition to six other Mahdi militia leaders who have been detained since the beginning of October.
The operations against the Sunni fighters resulted in the capture of 33 cell leaders in Baghdad, it said.
Iraqi and US forces, reinforced by up to 10 brigades, are preparing a broad offensive against Sunni fighters and Shia militia groups focused on Baghdad in an attempt to quell sectarian violence that has killed tens of thousands of Iraqis last year.
The US military has regularly charged that al-Sadr's Mahdi Army is heavily involved in sectarian killing of Sunni Arabs in Baghdad and other regions of the country.
On Friday, security forces captured a top al-Sadr aide from Baghdad, Sheikh Abdul Hadi al-Darraji, the group's Baghdad spokesman.
He was detained in an early morning raid along with four others from a Shia religious site near Sadr City.