BAGHDAD (AP) – Shiite militants fought U.S. and Iraqi forces around Baghdad’s Shiite district of Sadr City early Saturday despite a call for calm by anti-American cleric Muqtada al-Sadr following the assassination of one of his top aides.

At least 13 Shiite militants died in the clashes, which erupted Friday night and tapered off early Saturday, the U.S. military said. Iraqi police reported seven civilians were killed as a result of the fighting between U.S and government troops and al-Sadr’s Mahdi Army militia.

Al-Sadr blamed the Americans and their Iraqi allies for the assassination Friday of one of his top aides, Riyadh al-Nouri, director of his office in the Shiite holy city of Najaf. Gunmen ambushed al-Nouri as he was returning home from Friday prayers.

A curfew was declared in Najaf to prevent a violent backlash by al-Sadr supporters, but it was lifted Saturday. In Sadr City, a U.S. statement said American soldiers used Abrams main battle tanks and drone-fired Hellfire missiles in support of troops who came under sniper and rocket attack while trying to erect concrete barriers in the area.

Two armored vehicles were damaged by at least 10 roadside bombs that exploded during the operation, but there were no casualties among the U.S. and Iraqi soldiers, the military said.

The U.S. said a total of 13 extremists were killed in the various encounters.

Iraqi police and hospital officials said the seven civilians died in one of the Hellfire missile strikes. They spoke on condition of anonymity because they are not allowed to release information. It was impossible to verify the reports independently. Sadr City is the principal stronghold of the Mahdi Army in the Baghdad area.

Government troops supported by the U.S. military have been fighting for nearly two weeks to seal off Sadr City, which has a population of about 2.5 million, after militants there fired rockets and mortars at the U.S.-protected Green Zone and other major targets.

According to officials in Sadr City, Sadrist militiamen received instructions from the movement’s headquarters in Najaf on Friday to avoid confrontations with Iraqi and U.S. forces, and not to fire on them unless they attempted to penetrate deep into Sadr City.

The Sadr officials who spoke on condition of anonymity due to the sensitive nature of the issue, said the movement was concerned that clashes with security forces were turning into a war of attrition which rival Shiite groups were exploiting to weaken the Sadrists.

A ban on entering and leaving Sadr City was supposed to have been lifted on Saturday. Police announced that one of the entrances had been opened to motor traffic.

Army patrols used loudspeakers to warn residents to keep off the streets, saying the rebels had planted roadside bombs which needed to be cleared by the security forces.

The conflict in Sadr City is part of a major power struggle within the Shiite community ahead of provincial elections expected this fall.

In the southern port of Basra, Iraq’s second largest city, the Iraqi army said it had started carrying out “the second phase of operations” by conducting a sweep of the city’s Qibla district, looking for illegal weapons, ammunition and wanted criminals.

Last month, Prime Minister Nouri al-Maliki ordered the military to confront Shiite militias and gangs in Basra, but the offensive quickly faltered amid fierce resistance.

Elsewhere, Iraqi soldiers acting on tips from detained Shiite militiamen found 14 bodies that had been buried in a field south of Baghdad, officials said. It was the second discovery this week of mass graves in the area, raising to 44 the number of bodies located there.

The victims are believed to have been killed more than a year ago as part of a cycle of retaliatory violence between Shiites and Sunnis that has since ebbed.