U.S. soldiers in Sadr City have faced assaults by militiamen wielding rocket-propelled grenades on roads laced with bombs and have responded with airstrikes and tank fire.
The military says it makes every effort to avoid civilian casualties; however, residents are often caught in the crossfire.
The number of civilian deaths reported by the Iraqi government for April was 969, the highest since August, when 1,773 were recorded killed. At least 28 Iraqi soldiers and 69 policemen also were reported killed. Officials at two hospitals in Sadr City alone said they had received 321 bodies in the last month.
Prime Minister Nouri Maliki, a Shiite, lashed out Wednesday at the militiamen, whom he accused of using civilians as human shields. He vowed to disarm all militias, Sunni or Shiite.
"There is only one army, and that is the army of the state," he told reporters at a news conference broadcast on state-run television.
A total of 354 suspected militants were killed and 1,270 arrested in Baghdad last month, according to government figures.
Sadr's followers say they are being unfairly singled out while Maliki's political allies are permitted to maintain armed wings. They accuse their Shiite rivals of using the crackdown to weaken the chances of Sadr's followers in provincial elections scheduled for Oct. 1.
Two of the latest attacks against U.S. troops occurred in Baghdad, where three soldiers were killed in two bombings Wednesday. A fourth soldier was killed in a similar attack in Nineveh province, north of the capital.
The number of U.S. military deaths in April was the highest since September, when 65 U.S. service members were killed. In April 2007, 104 died, according to figures compiled by the independent website icasualties.org.
At least 4,063 U.S. personnel have been killed since the start of the Iraq war in March 2003, according to the site.
The level of violence has been inching up since January, after a 60% drop in attacks nationwide in the second half of last year, according to U.S. military figures.
Commanders attribute the 2007 drop-off to the U.S. troop buildup, the decision by tens of thousands of Sunni tribesmen and former insurgents to fight against extremists, and a unilateral cease-fire declared by Sadr in August.
Three of the additional combat brigades deployed for the buildup have returned home. U.S. commanders had been counting on Sadr's truce to allow them to focus on keeping up the pressure on Sunni militants as more troops left.
