About 600 fighters and 16 leaders of the radical Shia militia, the Mehdi Army, have been captured by security forces in Iraq, the US military says.
Many Mehdi Army fighters have melted out of sight
The statement said 52 operations had been conducted in 45 days targeting the militia, which is loyal to Najaf-based cleric Moqtada Sadr.
Sunni extremists were also the focus of the crackdown, the US military said.
US and Iraqi forces are currently preparing for a broad offensive in the strife-torn Iraqi capital Baghdad.
In other developments:
- Five Iraqi police were reported to have been killed in a gun battle in Mosul
- At least three people died in car bomb attacks in central Baghdad
- The deaths of three US soldiers were announced
- Thirty Palestinian men were abducted in Baghdad by gunmen wearing police uniforms. They were later released.
- A civilian helicopter crashed in a Sunni area of Baghdad, killing five people. Unconfirmed reports said it had been shot down.
A UN envoy in Iraq said the country was sliding "into the abyss of sectarianism" after two car bombs killed 88 people in a Baghdad market on Monday.
"These deplorable outrages again underscore the urgent need for all Iraqis to reject violence and together choose the path of peace and reconciliation," Ashraf Qazi said in a statement.
'Responsible for attacks'
The military said five of the Mehdi Army leaders were detained in the pro-Sadr bastion, Sadr City. One senior figure was killed in a raid.
"The detainees are responsible for attacks against the government of Iraq, Iraqi citizens and coalition forces," 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."
In addition, 33 Sunni extremist cell leaders were detained in Baghdad, the statement said.
The statement said they were "responsible for foreign fighter facilitation, car bomb facilitation, and propaganda operations".
Correspondents say the Mehdi Army has up to 60,000 fighters.
The BBC's Andrew North in Baghdad says it is still not clear how significant the senior Mehdi Army figures now in custody are.
But this appears to be the beginnings of a harder line on this widely feared Shia militia, he says.
Sadr's group has spearheaded anti-US military action in the past
In the past, the Iraqi government has been criticised for turning a blind eye to Mehdi Army activities for political reasons.
A spokesman for the movement would not confirm the numbers detained, but he said they were now seeing Iraqi and US raids almost every day.
Police are still finding dozens of bodies across the capital every day, most of them believed to be the victims of sectarian death squads.
Many Iraqis remain deeply sceptical that the Mehdi Army will be broken up, our correspondent says, and those fighters who have gone to ground are believed to have hidden their weapons, ready for future confrontations.