Radical cleric Muqtada al-Sadr said he will not enter any political process that would allow U.S. forces to remain in Iraq.

Al-Sadr also denounced U.S. Defense Secretary Robert Gates as a terrorist and said he will never work with Iraq's occupiers.

On Friday, Defense Secretary Robert Gates said al-Sadr was a significant political player due to his large following in Iraq's Shiite community. Gates also called on al-Sadr to take part in the political process.

Al-Sadr's Mahdi Army militia has been battling U.S. and Iraqi government troops for more than two weeks. Hundreds have died in the fighting.

But in a statement Saturday, al-Sadr said the Iraqi government must insist on the withdrawal of foreign forces if it wants peace in the country.

At least 13 Shiite militants died in the latest round of 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 when by 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.

A ban on entering and leaving the district was supposed to have been lifted on Saturday. But eyewitnesses said the blocked-off entrances were briefly opened at 9 a.m., only to be closed again after about 10 minutes.

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