Followers of Shiite cleric Moqtada al-Sadr attend open-air Friday prayers in the Shiite stronghold of Sadr City. (Karim Kadim/AP)

Iraqi Shiite cleric Moqtada al-Sadr said Tuesday that he would freeze the activities of his militias, potentially pulling back thousands of fighters from the battle against Islamic State militants.

Sadr claimed to have taken the decision as a “show of goodwill” following reports of atrocities by Shiite militia groups, many of which are not under his control. Analysts said, however, that other strategic motivations may have prompted the move.

The cleric’s order covered the Peace Brigades and the Promised Day Brigade, both of which have their roots in the Mahdi Army, the notorious Sadr militia that was responsible for thousands of attacks on U.S. troops during the Iraq war.

The Peace Brigades is among the largest of the Shiite militias that have proliferated since the Islamic State rapidly captured a vast swath of Iraq over the summer. The Peace Brigades stepped in to plug security gaps as Iraq’s army struggled to cope with the Islamic State’s onslaught, but the rise of the militias has been accompanied by reports of mass killings and forced displacement.

In the latest incident, a Sunni tribal leader and eight members of his entourage were killed Friday night, sparking outrage among Sunni politicians — who are now boycotting parliament in protest.

Iraq is suffering because of the actions of the “brazen militias,” Sadr said in a statement. “The latest period has affected Iraq by empowering the militias and their violence.”

He called on political parties to “show restraint” and urged them not to withdraw from the political process.

New York-based Human Rights Watch said last week that escalating abuses by Shiite militias in Sunni areas potentially amounted to war crimes.

Hussein al-Khafaji, a Sadrist lawmaker, said Sadr’s move meant that all of the cleric’s militia members would withdraw from battle, though a decision may be made for some to remain in Samarra to protect the Shiite shrine in that city.

“Other than that, they will retreat,” Khafaji said. “Sadr has taken the decision that the army can fill the space.”

Dia Asadi, another Sadrist lawmaker, told Al Sumaria television that Shiite fighters already had largely withdrawn. “They will remain on high alert when they go back to their normal lives,” he said.

Sadr may have frozen the activities of his militias to assess their loyalty, said Phillip Smyth, a researcher at the University of Maryland who specializes in Shiite Islamist militarism. Iran, which has little influence over the cleric, who plays to Iraqi nationalist sentiment, is probably attempting to encourage a splintering in Sadr’s ranks, Smyth said.

“It’s a point of turmoil, and he’s trying to draw out those potential splinters,” the researcher said. “Meanwhile, Iran is looking to its own ends and wants to maintain its influence.”

Smyth estimates the strength of the Peace Brigades at 25,000 to 40,000 fighters. It was formed with much fanfare in June, when tens of thousands of militiamen paraded with assault rifles, rockets and improvised explosive devices through the Baghdad neighborhood of Sadr City.

The Promised Day Brigade was created when Sadr froze the activities of the larger Mahdi Army in 2008.

Although Shiite militias have been leading the fight against the Islamic State, Sadr’s affiliates have played a less prominent role. When they were formed, he said their main role was to protect Iraq’s shrines, though the groups have taken part in major operations, including the capture of Jurf al-Sakhar, a Sunni area south of Baghdad, from Islamic State militants.

Sadr’s announcement on Tuesday came after comments last month in which he called for the country’s militias to be reined in, saying his followers were at the army’s command.